New Layered Compounds for High Temperature Superconductivity

 

This essay is a continuation , or update , of a previous Chemexplore web page , “Antiferromagnetic Induction in High Temperature Superconductors”. The new compounds discussed there would have alternating metallic layers for conduction and superconduction , and nonmetallic , but strongly antiferromagnetic layers for induction of an antiparallel spin rgime in the mobile free electrons above EF – the Fermi level – in the metallic bond of the former layers . This induced antiparallel spin orientation in the free electrons would permit them to couple together magnetically to form the Cooper pairs required for superconductivity , as discussed in the Antiferro web page . A variety of metallic and antiferromagnetic components were proposed for the layered composites . More materials for them will be examined in this survey ; several could possibly provide superconductor candidates with extraordinarily high transition temperatures .

 

Rhenium Trioxide as the Metallic Substrate

 

The bronzes would be excellent metallic substrates in the new layered composites (the references are listed at the end of this web page . Underlined blue hyperlinks can be clicked when online to download the PDF or HTML file , which will open in a new window) . Bronzes have the general formula MnXO3 , where M is a large electronically inert metal cation (usually of an Alkali metal such as Na1+ or K1+) . X is the smaller electronically active – usually Transition metal – atom , and O is an oxygen atom (not an oxide anion ; the X–O bonds are covalent , not ionic) . The value of the subscript n for Mn always lies between zero and unity . Ten elements (bronzes , Dickens and Whittingham) are generally considered to form bronzes : Ti , Zr , V , Nb , Ta , Cr , Mo , W , Re , and U , although the tungsten bronzes are the best known and most studied of the class .

The bronzes typically have bright colors and a metallic luster . Their electrical conductivities can be remarkably high for a chemical compound ; at higher doping levels of M (which donate their valence electrons to the metallic bond – conduction band – over the X–O lattice framework) the conductivites of the bronzes are comparable to those of many elementary metals and alloys . For example , the cubic tungsten bronze Na0.85WO3 has an ambient conductivity of around 70,000 ohm-1cm-1 (bronzes , Shanks , Sidles , and Danielson) . The higher the electrical conductivity of the metallic substrate the better , so the bronzes would be a good source of the metallic component in the proposed layered composites .

The MnXO3 bronzes have the perovskite crystal structure , some with a cubic symmetry , but more often with a distorted variation of that , in which the X–O layers are shifted laterally away from the cubic form shown below :

Green spheres : large M cations ; small red spheres : X metal atoms ; blue spheres : oxygen atoms .

The compound rhenium trioxide , ReO3 , is also considered to be a type of bronze (bronzes , Dickens and Whittingham) ; it has a crystal structure like that of the perovskites , but without a large central M cation :

Rhenium trioxide is a reddish-purple solid :

This image was copied from the Wikipedia web page , “Rhenium Trioxide”. I thank its author and Wikipedia for implied permission to reproduce it here .

Rhenium trioxide is thermally stable up to around 400 C , at which point it decomposes by disproportionating into ReO2 and Re2O7 . This might be a problem in the layering process , which conventionally is carried out by a shake-'n-bake technique at relatively high temperatures . The two components metallic and antiferromagnetic – would be thoroughly ground together in a mortar with a pestle (a procedure used by the alchemists of the Middle Ages !) , pressed into a cylindrical pellet , then heated in a furnace at an elevated temperature , preferably under an inert atmosphere of pure , dry nitrogen or argon . The pellet would be cooled to room temperature , ground up again , and the process would be repeated until the composite has a uniform composition and crystal structure , as shown by X-ray analysis . The relatively low temperature of 400 C might prove to be insufficient to achieve a proper combination of the rhenium trioxide with the antiferromagnetic induction agent .

ReO3 is an attractive metallic substrate for the new layered composites because it has the highest ambient electrical conductivity of all the bronzes : 149,300 ohm-1cm1, comparable to that of many common metallurgical metals (cf. nickel , 144,300 ohm-1cm1 at 293 K) . Its parent element rhenium , with a hexagonal close-packed (hcp) crystal structure , has a much lower ambient electrical conductivity , 58,140 ohm-1cm1 at 273 K . Clearly , the oxygen atoms linking the rhenium atoms in ReO3 are providing electrons for its metallic bond .

I have discussed the electronic structure of rhenium trioxide in other Chemexplore web pages , suggesting it has a bilayer metallic bond . In this picture , the 2py,z electron pairs in the oxygen linking atoms provide the paired electrons below EF in the metallic bond , and the rhenium atoms' valence electrons (nominally 6s2, but actually displaced into the nearby 6py,z orbitals) are above EF in its upper layer . My version of its electronic structure is presented below , in the form of a sketch derived from the picture VB (valence bond) theory :

The bilayer metallic bond in ReO3 is predicted by picture VB to be the Re 6py,z – O 2py,z p XO (XO = crystal orbital = metallic bond = conduction band)

The pi XO proposed as the metallic bond in ReO3 is both isotropic and nodeless (it's somewhat like the pi XO in graphite) , making rhenium trioxide a true metal , as are all the common metallurgical metals . Like them , it should have an inverse temperature–electrical conductivity relationship , and it does . A true metal with a bilayer metallic bond and a high ambient electrical conductivity is exactly the type of metallic solid we want as a precursor material for high temperature superconductivity . Unfortunately – wouldn't you just know it ! – rhenium is a rather rare element , and it and its compounds , including ReO3 , are very costly (M.J. Magyar ,“Rhenium”, PDF , 200 KB) .

The bilayer structure of rhenium trioxide's metallic bond provides it with a very rich population of mobile , free electrons above EF , resulting in its remarkably high electrical conductivity . However , ReO3 never becomes superconducting (PDF , 258 KB) , even very close to Absolute Zero . As is typical of all of the bronzes (and also many of the common metallurgical metals) it displays Pauli paramagnetism , which is caused by the randomly-oriented magnetic spins of the singlet electrons above EF in the metallic bond . If we can somehow cause these mobile , free electrons to adopt an antiparallel-ordered spin rgime , they might be able to magnetically couple together and condense into the Cooper pairs required for superconduction . Then ReO3 could become a superconductor , maybe even at a significantly high temperature . That's where the adjacent sandwiching layers of the nonmetallic , but strongly antiferromagnetic induction agent , come into play .

 

Nickel Oxide as an Antiferromagnetic Induction Agent

 

Nickel is one of the more abundant metallic elements in the Earth's crust , and its lower-valent oxide , nickel(II) oxide , NiO , is an important industrial commodity , being a precursor to nickel metal , which is used in a wide variety of commercial alloys such as stainless steel and Monel . There is some confusion as to the exact identity of pure nickel(II) oxide . Commercial NiO is usually black , while chemically pure , stoichiometric NiO is a light green color . The Merck Index , for example , says of nickel hydroxide : “green nickel oxide ....... apple-green powder , ignited in air at about 400 C , absorbs oxygen and is converted into black nickelic oxide” [8th edition , 1968 , p. 728] . It would seem that pure NiO is sensitive to air oxidation at elevated temperatures , so when heated it must be protected by a blanket of nitrogen or argon . The black appearance of partly oxidized NiO is caused by the presence in its lattice of some Ni3+ cations ; black NiO is thus a mixed-valent compound of Ni2+ and Ni3+ . The nickel(II) 3d8 electrons can rapidly resonate between the Ni2+ and Ni3+ cations . Such a resonance is promoted by ambient energy , including incident light , which is thereby strongly absorbed by the nickel cations , making the solid look black . I found the following picture of green nickel oxide on the Web :

(found on the web page , Huayou Cobalt ; my thanks to the copyright owner)

There are two good reasons for carefully avoiding the use of black nickel oxide in these layering experiments . First , Ni3+ is a fairly strong oxidizing agent :

Ni3+ + e- -------------> Ni2+ ; E0red = 1.17 V .

Ni3+ would instantly oxidize the Re(VI) in ReO3 to Re(VII) when mixed with the trioxide :

Re(VI) – e- -------------> Re(VII) ; E0ox = – 0.768 V ;

Net reaction : Re(VI) + Ni3+ -------------> Re(VII) + Ni2+ ; E0T = + 0.402 V ; this reaction is predicted to be thermodynamically spontaneous at STP .

Obviously , such a depletion of the metallic bond electrons in ReO3 is highly undesirable . Second , as pointed out , black NiO is a mixed-valent compound with a pronounced resonance of the nickel 3d8 valence electrons . As discussed in the Antiferro web page , such a mixed-valence resonance degrades and even destroys the antiferromagnetic ordering rgime in the NiO by “churning up” the 3d electrons , which again – obviously – we want to avoid . Therefore , only chemically pure , stoichiometric NiO (green , anhydrous) should be used in the layering experiments . And of course it would be essential to protect the NiO from air oxidation at elevated temperatures by conducting all the preparations under an inert atmosphere of pure , dry nitrogen or argon .

Nickel(II) oxide has the rocksalt crystal structure , which is slightly distorted from a cubic symmetry below 250 C ; above that temperature it is perfectly cubic :

Octahedrally coordinated nickel(II) cations are always in a high spin condition :

An electron in the dz2 orbital is known to be stereochemically prominent ; Transition metal cations with such valence electrons have a tendency to bulge outward , or elongate . This bulging effect usually results in elongated metal–ligand axial bonds in their compounds ; it's particularly pronounced in the copper(II) compounds [3d9] . In physics terms the phenomenon is called the Jahn-Teller effect . I think that the Ni2+ cations in NiO are slightly affected by the Jahn-Teller effect below 250 C because of the singlet electrons in their 3dz2 orbitals ; possibly the thermal energy in the solid above 250 C mixes all of the 3d electrons , reducing the Jahn-Teller effect on the nickel cations , making them more spherical and permitting the atoms to assume a cubic symmetry in the NiO lattice . At the same time the thermal mixing of the 3d electrons degrades their antiferromagnetic ordering , resulting in a transition to an “ordinary” Curie paramagnetism in the hot solid .

The antiferromagnetic property of nickel(II) oxide is of key importance in its potential application as an induction agent in the layered composites . Moore discusses the magnetic properties of NiO in his excellent overview of the physical chemistry of NiO . His Fig. 5.2(a) , p. 136 shows the magnetic structure of nickel oxide , as determined by the technique of neutron diffraction , with the 3d electron spins alternating from one direction to the reverse from one nickel cation to the next . Since pure , stoichiometric NiO is an electrical insulator , the Ni 3d electrons are pinnned on their respective atoms and their spins are locked in place by an oxide anion mediated electron superexchange via the latters' 2p orbitals [illustrated in Moore's Fig. 5.2(b) , p. 136] .

The magnetic susceptibility of nickel(II) oxide over a temperature range [unfortunately , from only 0 – 400 C] is graphed in Moore's Fig. 5.1 , p. 135 . For the reader's convenience I've reproduced his data in the following sketch :

Nickel oxide is appreciably antiferromagnetic at room temperature . Its Nel temperature , TN , is about 250 C (525 K , CRC Handbook of Chemistry and Physics , 87th edition , 2006 , p. 12-107) . If we can reasonably assume an effective degree of antiferromagnetic induction , i.e. sufficient to establish an antiparallel ordering in the metallic bond free electrons above EF , to be exerted at around TN , then NiO might become inductively effective at x 525 K = 263 K = – 10 C . Some additional cooling of the layered composite could then result in the condensation of the ReO3 antiparallel spin free electrons into Cooper pairs , and then ....... superconductivity !

As discussed in the Antiferro web page , antiferromagnetic induction from the NiO layer into the metallic ReO3 layer would be accomplished by electron superexchange from the nickel cations , through the oxygen atoms' 2p orbitals , into the rhenium atoms' XO free electrons . In effect , the metallic bond free electrons will mirror the antiferromagnetic – antiparallel – electron spin rgime of the nickel 3d valence electrons :

Note that while a fairly strong induction may be accomplished at a relatively high temperature , very likely more cooling of the composite will be necessary to complete the full condensation of the Cooper pairs and thus create a superconducting state in the material . Even so , a ReO3–NiO layered composite – if it can be successfully synthesized – might have quite a respectable transition temperature , possibly above 200 K . This tantalizing prospect should make it of considerable interest to superconductor researchers .

But would it be possible to “mate together” the two precursor compounds , ReO3 and NiO ? Both have cubic (or nearly so) lattices , although the Re–O–Re bond length (3.734 ) is somewhat shorter than that of Ni–O–Ni (4.1684 ) :

The implication is that the Re–O–Re covalent bonds might have to stretch a little in order to accomodate the fixed ionic radii of the nickel cations and oxide anions in the NiO layers .

The dissimilar natures of ReO3 and NiO are both an advantage and a disadvantage in the preparation of their layered composite . The former compound is covalent–metallic , and is stable only up to a relatively low temperature (400 C , dec.) . Nickel oxide is a refractory (m.p. 1984 C) , completely ionic , nonmetallic solid . Assuming the two materials can form a layered composite , their individual covalent ReO3 and ionic NiO layers should remain cleanly separated , with essentially no diffusion of high spin paramagnetic Ni2+ cations into the ReO3 layers . This is an important consideration , as such paramagnetic impurities could inhibit the formation of the Cooper pairs in the metallic layers .

The disadvantage of their two different natures is that it would probably be quite difficult , if not impossible , to prepare the ReO3–NiO layered composite from a direct combination (via a shake-'n-bake technique) of the two components . The ReO3 would decompose at a temperature likely well below that required for a proper diffusion of the component atoms in the mixture . We might instead try another procedure to avoid this problem .

The following redox reactions can be written to obtain a synthesis reaction :

2 Re(VII) + 2e- -------------> 2 Re(VI) ; E0red = 0.768 V ;

Ni0 – 2e- -------------> Ni2+ ; E0ox = 0.257 V ;

Net reaction : 2 Re(VII) + Ni0 -------------> 2 Re(VI) + Ni2+ ; E0T = 1.025 V ; this reaction is predicted to be thermodynamically spontaneous at STP (and possibly quite exoergic) .

The chemical reaction representing this redox reaction is :

Re2O7 + Ni0 + NiO -------------> 2 [ReO3–NiO] .

The rhenium heptoxide component of the mix , Re2O7 , has been described as “canary-yellow , very deliquescent , hexagonal crystals” (Merck Index , 8th edition , 1968 , p. 916) . It melts at 297 C , but begins to sublime at ~ 250 C (according to the CRC Handbook of Chemistry and Physics , 87th edition , “Physical Constants of Inorganic Compounds” [PDF , 1086 KB] , it melts at 327 C and boils at 360 C) . Re2O7 dissolves in , and reacts with water to form perrhenic acid , HReO4 .

The synthesis of the layered composite could be carried out in several steps :

First , a quantity of pure , anhydrous , stoichiometric (green) nickel(II) oxide would be obtained , either purchased or prepared by the researcher . As noted above , most commercial NiO is black , thus containing the undesirable Ni3+ oxidizer impurity . The pure NiO would be ground to a very fine powder .

Second , equimolar quantities of the NiO , nickel metal powder , and rhenium heptoxide would be thoroughly ground together . The nickel metal powder should be very finely divided . For example , the product “Nickel flake , –325 mesh , 0.37 micron thick , 99.9% (metals basis)”, offered by Alfa-Aesar , might be suitable for the reaction .

Third , the mixture would be heated cautiously , since it contains a combination of a reducer (Ni0) and oxidizer (Re2O7) . If there was no appreciable exotherm , it might be possible to press some of the mixture into a cylindrical reaction pellet in a press . The reaction pellet would be heated in a muffle furnace to no more than ~ 350 C under an atmosphere of pure , dry nitrogen or argon . I'm just guessing at this third step , as the reaction might commence at a much lower temperature .

Fourth , the reaction pellet would be cooled to room temperature , ground to a fine powder , repelletized , and reheated .......... and so on , until a homogeneous reaction product (as shown by X-ray diffraction) was obtained .

As noted above , rhenium trioxide is a metastable compound , which readily disproportionates into the two stabler valence states , Re(IV) and Re(VII) . NiO could even catalyse or otherwise accelerate its disproportionation , especially when heated :

3 ReO3 + NiO -------------> ReO2 + Ni(ReO4)2

An ambient synthesis of a layered ReO3–NiO composite might be carried out using highly reactive Re and Ni molecular precursors . Rhenium heptoxide , a covalent compound , dissolves in many organic solvents (the Merck Index says , “alcohol , ether , ethyl acetate , dioxane , pyridine”) . The zerovalent nickel reagent tetrakis(triethylphosphite)nickel(0) , [(C2H5O)3P]4Ni – a white , air-sensitive powder , m.p. ~ 106 C – is presumably soluble in various organic solvents , possibly including ethyl ether . Its highly reactive Ni(0) should be quickly oxidized by Re2O7 . An ether solution of rhenium heptoxide would be added slowly from a pressure-equalizing funnel under nitrogen to an ether solution of the nickel reagent in a round-bottom flask , under nitrogen , with rapid mechanical or spinbar stirring . The insoluble (ReO3)2–NiO combination should promptly precipitate as fine – probably black – microcrystals :

Re2O7 + [(C2H5O)3P]4Ni ------ (ethyl ether , 20 C) -------> (ReO3)2–NiO (c) + 4 (C2H5O)3P

This product would be filtered from the solution , washed with dry ether , and dried at room temperature in a dessicating jar under vacuum . To avoid disproportionation of its rhenium(VI) , it should not be heated , but examined and tested “as is”. Its compressed disc could be used for electrical conductivity testing . Such a disc might resemble the “KBr discs” made in a laboratory press by organic chemists for the infrared spectroscopy analysis of an organic solid .

In an alternate procedure , the Re and Ni precursor reagents would be dissolved in two immiscible solvents : the Re2O7 in water (forming perrhenic acid , HReO4) , and the Ni(0) reagent preferably in a dense , water-immiscible organic solvent , such as chloroform . The upper water layer would protect the oxygen-sensitive Ni(0) reagent in the lower layer from the air . The 2 : 1 adduct would form as a sort of “inorganic polymer” at the water–chloroform interface , slowly precipitating from the liquid phase :

2 HReO4 + [(C2H5O)3P]4Ni -------------> (ReO3)2–NiO + H2O + 4 (C2H5O)3P

As a variation of this reaction , a small quantity of a nonionic surfactant , such as a polyethoxylated fatty alcohol (eg. Brij 30) , would be dissolved in both the water and chloroform components . Such detergents are known to act as crystal habit modifiers in the crystallization of solids from solutions . Their molecules are adsorbed on the sides of the growing crystals , inhibiting their sideways growth . The crystals then propagate lengthwise , resulting in the formation of long , acicular needles . These needles are very pure ; the crystallization process resembles zone refining in certain aspects . The needle-like crystals are very useful in electrical conductivity testing .

The ReO3–NiO [or (ReO3)2–NiO] layered composite , if successfuly synthesized by whatever procedure , would be subjected to various physical tests , including of course its electrical conductivity over a range of temperatures , and hopefully superconductivity . As to whether or not it would be a genuine layered compound , or merely an intimate mixture of fine-grained microcrystals of ReO3 and NiO , would have to be determined by high power microscopy and X-ray crystallography .

The reaction of rhenium heptoxide with zerovalent metal carbonyls could be quite general , and would lead to the synthesis of a variety of fascinating new solid state materials . For example , the carbonyls of nickel , cobalt , iron , and manganese could produce layered composites of (ReO3)2–MO , where M = Ni , Co , Fe , and Mn . Their corresponding oxides have antiferromagnetic rocksalt crystal structures with TN = 525 , 291 , 198 , and 122 K , respectively . The following Table lists nine Transition metal carbonyls which might reasonably be expected to react with rhenium heptoxide . Note that several have M–M covalent bonds :

Many other classes of zerovalent organometallic compounds are known , and might also readily react with rhenium heptoxide ; I pointed out tetrakis(triethylphosphite)nickel(0) as a possible substitute for the very toxic nickel carbonyl . Regrettably , the scope of such zerovalent organometallic chemistry is far too wide for the limited length of this web page .

 

Niobium Monoxide as a Metallic Substrate

 

Niobium monoxide , NbO , also has a bilayer metallic bond . Its ambient electrical conductivity is around 50,000 ohm-1cm-1, and it superconducts at Tc = 1.38 K . That of its parent element , niobium , is 65,789 ohm-1cm-1 at 273 K ; its Tc = 9.3 K . NbO is a refractory (m.p. 1940 C) black powder , but it has a silvery , metallic sheen as a fused button . Its cubic crystal structure was for a long time thought to be a defect rocksalt lattice . NbO is now recognized to have a metal cluster type of structure with covalent Nb–Nb bonds :

The electronic structure of niobium monoxide is discussed in the Solids web page . NbO's bilayer metallic bond was proposed there to be the niobium [4s2 – 4pz1 ] – oxygen [2s2 2p2z ] XO . A magnetic study of NbO revealed Pauli paramagnetism in the solid , but no Curie paramagnetism , thus ruling out the possibility of having discrete Nb2+ cations in the lattice .

Niobium monoxide would seem to be another good metallic substrate for layering with nickel oxide . The anticipated NbO–NiO composite might have a crystal structure something like this :

The Nb–O–Nb and Ni–O–Ni bond lengths are about the same , so the NbO and NiO layers could fit reasonably well together in the composite .

NbO can be prepared by reproportionating Nb0 and Nb2O5 ; the reaction is carried out in an arc furnace . Similarly , the NbO–NiO layered compound would probably be best synthesized in an arc furnace under an inert argon atmosphere , given the refractory nature of the two components :

NbO (m.p. 1940 C) + NiO (m.p. 1984 C) ----------- (argon , arc furnace) -----------> NbO–NiO

Niobium monoxide is commercially available at a moderate cost , eg. from Alfa-Aesar .

 

Barium Plumbate (BaPbO3) as a Metallic Substrate

 

Metallic perovskites could in general provide many candidates for the superconducting layer in the composites . I've discussed the possible application of the highly conductive tungsten and molybdenum bronzes as such in the Antiferro web page . Another interesting compound in this regard would be barium plumbate , BaPbO3 , which is a perovskite with a “pseudocubic [orthorhombic] symmetry”. It's a true metal (its electrical conductivity at room temperature is 3448 ohm-1cm-1 , and at liquid helium temperature , 4.2 K , is 13,514 ohm-1cm-1) with a bilayer metallic bond , so it has the properties desirable for the layered composites . A suggested electronic structure of BaPbO3 , according to picture VB , is presented in the following sketch :

The bilayer metallic bond in the compound would appear to be the lead 7py,z–oxygen 2py,z p XO by this analysis . The dz2pz lobes of the sp3ds octahedral hybrid orbital might be more voluminous and “bulge out” more than the sp-type lobes , thereby lengthening the z axis of the cubic unit cell and making it orthorhombic , as is observed experimentally [Shannon and Bierstedt (barium plumbate)] .

The Pb–O–Pb bond length in BaPbO3 would be the cell constant , which has several reported values ; for example , 4.273 (Ellis et al. , undoubtedly for the cubic symmetry form reported by earlier researchers) . Longer cell constants were reported by Shannon and Bierstedt (barium plumbate) for their orthorhombic symmetry specimens . Using the Shannon-Prewitt crystal ionic radii for Pb(IV) , 0.78 (6-coordinated , i.e. octahedral) , and for oxide , 1.21 (2-coordinated , i.e. linear) , the Pb–O–Pb bond length is calculated as 2 x (0.78 + 1.21) = 3.98 , so I would tend to favor the 4.273 value . As this is longer than the Ni–O–Ni bond length in NiO (4.1684 ) , the Pb–O–Pb covalent bonds in the plumbate would have to shrink somewhat in order for the two different layers to fit together properly .

Shannon and Bierstedt (barium plumbate) have described the preparation of BaPbO3 by dry thermal (shake-'n-bake) and hydrothermal methods . Kodama and co-workers co-precipitated barium and lead oxalates from an aqueous solution , then calcined the oxalates in an oxidizing atmosphere to burn off the organic ligands , leaving a residue of BaPbO3 . Since the lead(IV) in the compound is a strong oxidizer , it follows that barium lead(IV) oxide should be synthesized under strongly oxidizing conditions . The combination of BaCO3 with PbO under a flowing atmosphere of pure oxygen (eg. Lee and co-workers) would be a straightforward route to BaPbO3 :

BaCO3 (99.9%+ pure , m.p. ca. 1450 C , dec.) + PbO (99.9%+ pure , m.p. 888 C) --------- (heat under a pure , flowing oxygen atmosphere) -----------> BaPbO3 + CO2 (g) .

Both barium carbonate and lead(II) oxide are commercially available as very pure reagents ; BaCO3 is extensively used in the preparation of high temperature superconductors such as YBCO . The flowing oxygen atmosphere (also used in the YBCO synthesis) oxidizes the lead(II) to lead(IV) and helps to drive the reaction to completion [Le Chatelier's Principle] by sweeping the by-product carbon dioxide gas out of the reaction zone .

The fusion of litharge (PbO) with sodium peroxide [ACS , 93% purity , m.p. 460 C (dec.)] might be a second possible oxidative procedure for the preparation of BaPbO3 :

Na2O2 + PbO ----- (fuse together) ------> Na2PbO3 (sodium metaplumbate) ----- [BaCl (aq)] ------> BaPbO3 (c) + 2 NaCl (aq) .

The following redox reactions suggest that barium plumbate might be prepared via oxidation of lead(II) to lead(IV) by hydrogen peroxide in aqueous alkaline solution at room temperature :

HO2- + H2O + 2e- -----------> 3 OH- ; E0red = + 0.878 V ;

PbO + 2 OH- – 2e- -----------> PbO2 + H2O ; E0ox = – 0.247 V ;

Net reaction : PbO + HO2- -----------> PbO2 + OH- ; E0T = 0.631 V ; this reaction is predicted to be thermodynamically spontaneous at STP .

Lead(II) hydroxide , Pb(OH) , is a white , water-insoluble solid that dehydrates to PbO at ~145 C . The addition of a solution of sodium hydroxide and hydroperoxide [Na+ HO2- , from the reaction :

NaOH (aq) + H2O2 (30% aq) -----------> Na+ HO2- + H2O] ,

to aqueous Pb2+ could precipitate the unstable compound Pb2+(OH-)(HO2-) ; the hydroperoxide anion would oxidize the Pb2+ cation to Pb(IV) , thus forming metaplumbate , O=Pb(OH)2 :

Pb(NO3)2 (aq) ----- (add 4 NaOH + H2O2 ) ------> Na2PbO3 (aq) + 2 NaNO3 (aq) + 3 H2O

----- [add Ba(NO3)2 (aq)] ------> BaPbO3 (c) + 2 NaNO3 (aq) .

This third possible method is an example of the “eco-friendly” chemie douce favoured by inorganic chemists these days , and would be the preferred route to BaPbO3 if it was successful .

Pb(IV) is a relatively strong oxidizing agent . Lead dioxide is the oxidizer in commercial lead-sulfuric acid batteries , for example as found in most cars and trucks :

PbO2 + SO42+ + 4 H+ + 2e- -----------> PbSO4 + 2 H2O ; E0red = 1.6913 V .

Lead(IV) should be able to oxidize Ni2+ to Ni3+ (E0ox = – 1.17 V) , making NiO unsuitable for use as the antiferromagnetic induction agent with BaPbO3 . Copper(II) , with a higher E0ox = – 2.4 V [to Cu(III)] , would be compatible – in the redox sense – with Pb(IV) . CuO is antiferromagnetic , with TN = 230 K . Inclusion of Cu2+ in the reaction mixture with the Ba2+, with additional hydroxide , would result in the co-precipitation of the gelatinous , blue Cu(OH)2 with the BaPbO3 . Filtration of the combined material , followed by washing , drying , and calcining (> 200 C , maybe as high as 800 C) under N2 or Ar , could produce the layered composite BaPbO3–CuO .

The big question in the synthesis of all these new composite materials is whether or not layered structures can actually be formed between the metallic component and the nonmetallic , antiferromagnetic induction agent . Quite complex layered lattices have been constructed by the combination of simple inorganic precursors in shake-'n-bake and chemie douce procedures ; the high temperature superconductors YBCO and BSCCO-2212 are good examples of such materials :

I pointed out in the Antiferro web page that BSCCO-2212 (Tc ~ 110 K) was a good example of exactly the type of compound we were aiming for in these layering experiments : the mixed-valent Bi(III,V) metallic layers (formally Sr2Bi2O5+x ) are sandwiched between alternating layers of the nonmetallic , but antiferromagnetic Ca2Cu3O5 . It's interesting to note in this regard that Arjomand and Machin prepared CaCuO2 back around 1975 , and found that it was only feebly antiferromagnetic : 1.72 BM (Bohr magnetons) at 300 K , and 1.63 BM at 80 K . [The spin-only magnetic moment m for n unpaired electrons , m = [n (n+2)] , so the Cu(II) 3d9 singlet electron in CaCuO2 should produce in it a magnetic moment of 1.73 BM at room temperature] . Perhaps the Sr2Bi2O5+x metallic layers could have a significantly higher Tc by intercalating them with a stronger antiferromagnetic induction agent , i.e. one with a high Nel temperature , TN . Arjomand and Machin found that the compound La2CuO4 (TN ~ 315 K) was strongly antiferromagnetic , even at room temperature : 0.69 BM at 300 K , and 0.34 BM at 80 K . It could therefore be a more effective antiferromagnetic induction agent for Sr2Bi2O5+x than the weaker Ca2Cu3O5 .

The sintering together at 900–1000 C of BaPbO3 and YBCO by Lee and co-workers failed to produce a new compound with alternating Pb–O and Cu–O layers :

“The composites included large-sized YBCO grains embedded in a BPO [BaPbO3] matrix” (p. 413) .

Their experimental results remind us that the attempted synthesis of a BaPbO3–CuO composite with alternating Pb–O and Cu–O layers using a shake-'n-bake (dry thermal) procedure could result only in a sintered mass composed of intimately mixed grains of the two precursors .

 

Epitaxial LaNiO3–LaFeO3 (and Other) Layered Composites

 

The best general method for creating the layered composite materials contemplated in this study is probably one or other of the chemical vapour deposition (CVD) techniques . The electronics industry has become highly reliant on CVD for the production of doped silicon , germanium , and related semiconductors . Since I'm mostly unfamiliar with CVD technology I'll restrict my discussion in this section to the chemistry of several compounds that might be used to fabricate metallic-antiferromagnetic layered composites in the form of epitaxial thin films .

Lanthanum nickel(III) oxide , LaNiO3 , has been known for a long time as a metallic solid , but more recently it has been the subject of much study as a thin film component . It is described as having a pseudocubic perovskite crystal structure , with a rhombohedral distortion at room temperature ; it .......“undergoes a rhombohedral–cubic transition at 940C and decomposes above 1120C” (Obayashi and Kudo) . Its pseudocubic unit cell dimension is a = 3.84 (Lisaukas and co-workers , thin film component) . Various different ambient electrical conductivity values have been reported for LaNiO3 : 29,400 ohm-1cm-1 (Lisaukas and co-workers) ; only ~ 100 ohm-1cm-1 (Obayashi and Kudo) ; and around 6000 ohm-1cm-1 for epitaxial films on a SrTiO3 substrate (Zhang and co-workers) . There is a general consensus that LaNiO3 has a metallic conductivity” ; that is , it has an inverse temperature–electrical conductivity relationship . This means it's a true metal , which is a requirement for high temperature superconductivity in a metallic component (among other factors) .

There also seems to be a general agreement that LaNiO3 exhibits a weak Pauli paramagnetism (eg. Sagoi and co-workers) , resulting from the spins of the mobile , free electrons above EF in its metallic bond . There is no indication of any Curie paramagnetism in the compound , that would be produced by the spins of singlet , unpaired 3d electrons in the nickel kernels . From a consideration of these physical properties of LaNiO3 we can sketch its possible electronic structure , according to the picture VB theory , as follows :

From this sketch we see that the metallic bond in LaNiO3 originates from the nominally 3d7 nickel valence electrons , which have been promoted up into the frontier 5 s-p orbitals . The 5s orbitals have the wrong shape and symmetry to overlap properly with the oxide anions' 2py,z orbitals ; however , the nickel 5py,z orbitals can overlap with them successfully to form the Ni 5py,zO 2py,z p XO , which is the proposed metallic bond in the solid . Nevertheless , most of the electron probability density will remain in the 5s orbitals , leaving only a small percentage as a trickle into the 5p orbitals . This suggests that while LaNiO3 indeed has a bilayer metallic bond , it's only a low density one , and so it probably isn't one of the better metallic compounds for the layering process . No doubt about it , rhenium trioxide with its high electron density and superb electrical conductivity is the premium metallic solid for synthesizing layered composites . I'm proposing LaNiO3 for investigation in this regard , though , because its crystal structure is somewhat similar to the suggested antiferromagnetic component of the materials , LaFeO3 . The Shannon-Prewitt crystal ionic radii of octahedrally-coordinated Ni3+ and Fe3+ cations are almost identical (0.56 and 0.55 respectively) , so there should be a reasonably good fit between the LaNiO3 and LaFeO3 layers in the composite .

Lanthanum iron(III) oxide is also a perovskite with a distorted symmetry , in this case orthorhombic (a = 5.557 , b = 5.565 , and c = 7.854 , Lning and co-workers ; Grepstad and co-workers reported a smaller lattice parameter , 3.932 , for bulk LaFeO3) . The large unit cell dimensions suggest that the Fe3+cations have an outer octahedral coordination (sp3d2) by the oxide anions , in contrast to the inner octahedral coordination (d2sp3) of the Ni3+cations in LaNiO3 [picture VB sketch above ; a = 3.84 , much shorter] . The relatively high magnetic susceptibility of LaFeO3 (Ahmed and El-Dek) indicates that its Fe3+cations' 3d5 valence electrons are in a high spin state :

The long c axis in the crystal suggests the possibility of Jahn-Teller distortion in it ; this could be produced by a high spin electron in the iron atoms' 3dz2 orbitals . The dz2 electrons are stereochemically quite prominent , and cause a pronounced bulge in the affected metal cation , making it appear oval in shape rather than spherical , as is normally the case .

The 3dz2 electrons might also be the source of the antiferromagnetism in LaFeO3 . Electrons in the other d orbitals (xy , xz , yz , and x2y2) tend to be located close to the atomic kernel , while the dz2 electrons are more spatially extended , and so could interact via superexchange more easily with the p orbitals of adjacent oxide anions . It should be noted in this regard that LaFeO3 is considered to be a canted antiferromagnet ; the spins of the iron 3d electrons responsible for its antiferromagnetism are tilted at a slight angle to the plane of the FeO atoms (Lning and co-workers , Fig. 1A , p. 214433-2 , magnetic structure of bulk LaFeO3 ; Peterlin-Neumaier and Steichele determined this tilting to be ~12 in the a-b plane and ~18 in the a-c plane) . Lning and co-workers found that the AFM axis of LaFeO3 could be tilted as much as 45 with respect to the major crystal axis of the substrate (usually SrTiO3) on which its film is grown . How this canting might affect the superexchange interaction of the Fe3+ cations in LaFeO3 with the LaNiO3 metallic bond free electrons can't be predicted at this point .

LaFeO3 is of particular interest as a possible antiferromagnetic induction agent because of its extraordinarily high Nel temperature . There are several literature values for its TN : 750 K (CRC Handbook of Chemistry and Physics ; Koehler and Wollan) ; Grepstad and co-workers mention 740 K for bulk LaFeO3 , but found lower values for epitaxially-grown LaFeO3 films : 645 K (as-grown , with strain) , and 610 K (relaxed films , strain-relieved) . Ahmed and El-Dek found that doping LaFeO3 with increasing mole fractions of Ca rapidly degraded TN in the doped composites : from TN = 740 K in La0.95Ca0.05FeO3 to TN = 670 K in La0.45Ca0.55FeO3 [their Fig. 4 , p. 32] . The calcium-doped composites are mixed-valent Fe(III)Fe(IV) compounds in which there is expected to be an ultrafast 3d5 valence electron resonance over the 3d4 Fe(IV) base cations . Such a resonance would churn up the iron electrons responsible for the antiferromagnetic structure , and thereby destroy it . I pointed out in the Antiferro web page that this same sort of mixed-valent resonance was a mixed blessing in , for example , the HTS cuprates with mixed-valent Cu(II)Cu(III) . On the one hand , it was required to unpin the frozen Cu(II) 3d9 electrons , making them the mobile , free electrons that condense into the Cooper pairs in the CuO bilayer metallic bond ; but on the other hand , it degrades the antiferromagnetism required to give those 3d9 electrons the antiparallel ordering they need for the magnetic coupling . The solution to this dilemma could very well be this present scenario : keep the metallic and antiferromagnetic components separate , but in very close proximity in the layered composites , and coupled together electronically via superexchange .

There is general agreement in the literature that pure , stoichiometric LaFeO3 is essentially an insulator (as we can quickly see from its picture VB sketch above ; there are no electrons in any frontier orbitals in it) ; for example , Jacobson found that the conductance of an amorphous LaFeO3 film prepared by a pulsed laser deposition method was ~ 3 x 10-7 ohm-1 . It can be made semiconducting by removing some of its lattice oxygens , or by cation doping ; in both techniques mixed-valent iron is produced , which results in valence electron hopping from iron to iron atom over the oxide anions via superexchange ; the doped compound is then a hopping semiconductor .

Even the lower TN values in epitaxial LaFeO3 films are still quite impressive and make the compound attractive for investigation as an antiferromagnetic induction agent . As mentioned above for the nickel oxide , if we assume an appreciable antiparallel ordering effect in the metallic bond free electrons at ~ TN , we might in practice observe an inductive effect in the LaNiO3 layers at around 300 K , room temperature . That may be somewhat optimistic ; but I think that a strong , effective induction (i.e. HTS) at 200+ K is not unrealistic for the LaNiO3LaFeO3 epitaxial film composite .

Some very solid evidence in favour of the antiferromagnetic induction concept has come from exchange bias physics , for example the considerable work of Scholl , Nolting , and co-workers . One of their most interesting experiments involved the formation of composites of cobalt metal (2.5 nm thick , with a protective outer coating of 1 nm of platinum) on top of LaFeO3 (40 nm thick , grown on a SrTiO3 substrate) , as epitaxial films . The ferrite was shown , by rather sophisticated analytical techniques , to have transmitted its antiparallel ordering rgime to cobalt's metallic bond mobile , free electrons , thus converting the normally ferromagnetic cobalt into an antiferromagnetic state . It would be interesting to see if the cobalt layer in this sandwich became superconducting if sufficiently cooled . Because it has a direct Co–Co metallic bond (monolayer) , which is therefore adversely affected by the Fermi-Dirac distribution , we can predict that Tc in the layered cobalt would be very low , fairly close to Absolute Zero . Nevertheless , any superconductivity observable in it would be a notable confirmation of the antiferromagnetic induction concept .

Exchange bias was discovered in 1956 , and is currently a very active field of research in solid state physics . It's an important phenomenon in electronics , for example being applied in the magnetic read heads of computer hard drives and in spintronic memory (RAM) . The most common technique for constructing such ferromagneticantiferromagnetic exchange bias epitaxial film composites is that of molecular beam epitaxy (MBE) . A survey of this physics research field is well beyond the scope of this limited (and chemistry-oriented !) web page , although several of the references cited below discuss exchange bias and its applications in some detail .

MBE experiments are described in several acccessible papers by Logvenov and co-workers , to which the interested reader is referred . In fact , I became aware of the possibility of using MBE specifically to prepare nanoscale layered composites of complex solid state compounds after finding an abstract by Bozovic and Logvenov from a conference , held at Notre Dame University on June 1012 , 2010 , Workshop on the Possibility of Room Temperature Superconductivity and Related Topics. The following is a reproduction of their abstract from that web page :

Artificial Superlattices Grown by MBE : Can We Design Novel Superconductors ?

 Ivan Bozovic and G. Logvenov

Brookhaven National Laboratory , Upton , NY 11973 .

“We wish to stress upfront that regrettably our group does not know (yet !) how to synthesize a room temperature superconductor . What we know how to do is to deposit atomically smooth layers of various complex oxides , using atomic-layer Molecular Beam Epitaxy (MBE) [1] . So far , we have worked with cuprates , manganites , and titanates , but the extension at many other oxides should be straightforward , since the hardware is in place and much of the experience seem transferable .

Some (many !) oxides can be grown nicely (i.e. , epitaxially) one-on-top-of-another , and this enables us to stack layers of different oxides , with disparate physical properties , on an extremely fine (sub-nanometer) scale , and form precise superlattices [1] . We will show some examples in this talk . In this way , we can synthesize novel , artificial materials that do not exist in Nature (i.e., which are only metastable) , and which could have novel and interesting properties .

Now , oxides exhibit an enormously wide range of properties , including extremes such as the strongest dielectrics , ferroelectrics , ferromagnets – and , last but not least , the strongest known superconductors with the highest Tc above 160 K – and hence the range of possible combinations is very large . Thus we need some theoretical guidance here – which pairs to try (first) . One possible choice is for one constituent to be a metal and the other to be an insulator in which excitons can live ; with such a superlattice we could achieve a physical realization of the 40-year old theoretical model of V. L. Ginzburg which he suggested [2] could provide a (quasi-2D) excitonic superconductor – with a very high Tc”.

[1] I. Bozovic et al. , Phys. Rev. Lett. 89 , 107001 (2002) ; Nature 421 , 873 (2003) ; Phys. Rev. Lett. 93 , 157002 (2004) ; P. Abbamonte et al. , Science 297 , 581 (2002) .

[2] V. L. Ginzburg , Phys. Lett. 13 , 101 (1964) ; Sov. Phys. JETP 20 , 1549 (1965) ; Rev. Mod. Phys. 76 , 981 (2004) .

My thanks to Drs. Bozovic and Logvenov (presumably the copyright holders for this abstract) . The published version of this conference paper is cited in Logvenov and co-workers , below .

The excitonic component referred to in the abstract derives from a concept in BCS superconductivity in which excitons “push together” singlet electrons into the Cooper pairs . Of course , Frenkel and Mott-Wannier excitons are well-known in solid state science . It was hoped that such excitons , in a properly designed molecular or crystal system , would be able to induce the metallic bond free electrons to couple together into Cooper pairs at exceptionally high temperatures , possibly even at room temperature . For example , W.A. Little theorized about such an “excitonically pumped” ambient superconductor polymer back in the early 1960s . To be honest , I've never understood how the excitons might accomplish the coupling . I believe the key to coupling them is in their orientation , since we know that the two electrons in a Cooper pair have a relative antiparallel orientation . Most normal metals exhibit Pauli paramagnetism , which is caused by the magnetic spins – having essentially random orientations – of the mobile , free electrons above EF in their metallic bond . If we can cause them somehow to assume an antiparallel ordering , they might be able to magnetically condense into Cooper pairs (this was discussed at greater length in the Antiferro web page) .

That's where the antiferromagnetic (AFM) induction layer enters the picture . If their metal atoms (with the antiparallel order electrons) can link to the atoms in the metallic layer via superexchange , then the lattter's electrons will mirror the antiparallel ordering in the former layer . Instead of the conventional exchange bias with AFM–ferromagnetic couples , this time it would be with AFM–Pauli paramagnetic couples . In that sense , superconductivity produced by such AFM exchange bias , which is considered to be a “spintronic” phenomenon – resulting from the electron spins – could be called spintronic superconductivity (as opposed to BCS phonon-mediated and excitonic superconductivity) . And MBE seems to be the ideal method for creating the AFM–metallic layered composites ! So I was both delighted and inspired by Bozovic and Lognenov's MBE research papers , and I decided to add this section about epitaxial layered composites to the web page .

It might be possible to create epitaxial film composites of LaFeO3 with the other metallic substrates discussed above : BaPbO3 , NbO , and of course the best one of all , ReO3 . It might also be possible to use other antiferromagnetic induction agents in the films , too , such as the nickel(II) oxide I reviewed earlier . NiO has already been examined in exchange bias studies and in MBE-generated nanofilms , so it should be suitable for use in the present context :

“NiO thin films are of technological importance as antiferromagnetic layers in exchange biasing applications and can be considered a model system to study” (interesting experiments , J. App. Phys. paper , p. 7268) .

Although NiO has a somewhat lower TN than LaFeO3 , it remains an attractive induction agent because of its straightforward , simple , no-nonsense magnetic structure , lacking the complexities (canting , helical screw structure , simultaneous antiferromagnetic/ferromagnetic ordering) of the iron-based antiferromagnets . I think it would provide a first-class epitaxial film composite with rhenium trioxide , the premier bilayer XO metallic solid .

Composites of LaFeO3 with the metallic solid SrFeO3 would be quite interesting . This latter compound is well known and has been considerably studied , both in bulk (pellets , single crystals) and in epitaxial films . The iron atoms in SrFeO3 have high spin 3d4 electrons , which are conventionally thought to be t2g3 eg1 . However , this might not be the case , as the following simple picture valence bond analysis suggests . Researchers have noted that (1) pure , stoichiometric SrFeO3 has a metallic conductivity” , i.e. it's a true metal which has a nodeless XO as its metallic bond ; (2) it's a perovskite with a cubic symmetry (a = 3.850 ) at all temperatures ; and (3) no Jahn-Teller distortion – which might be expected with an eg electron present – has been observed in SrFeO3 at any temperature . The picture VB sketch below takes all this information into account :

We see from this analysis that SrFeO3 is quite similar both in structure and properties to LaNiO3 (see its picture VB sketch above) . The metallic bond in SrFeO3 is similar to that of LaNiO3 : an attenuated Fe 5py,zO 2py,z p XO , from which we can predict that SrFeO3 will have only a poor sort of electrical conductivity like LaNiO3 . Bowen (Metallic perovskites) gives the conductivity of SrFeO3 as around 800 ohm-1cm-1 over a wide temperature range ; Lebon and co-workers (single crystals) measured it as ~ 1300 ohm-1cm-1 at 300 K . Oda and co-workers provide the value of approximately 1000 ohm-1cm-1 . This range of conductivities for SrFeO3 is probably comparable to that for LaNiO3 ; of course , the electrical conductivity of any particular sample is highly dependent on its preparation history and doping or other impurities .

Oda and co-workers comment ,

[SrFeO3] shows no Jahn-Teller distortion down to 4.2 K ..... against expectation if the Fe4+ ion were to take the high-spin electron configuration (t2g3 eg)” [p. C1-121] .

We see in the picture VB sketch above that the eg electron has been displaced by the strong Fe–O covalent bonds , which use its orbital in the formation of their inner octahedral d2sp3 hybrid orbital . This 3d4 electron enters the empty 5s frontier orbital (with leakage into the nearby 5p orbitals) , where it creates the Fe–O metallic bond , as noted . There are no eg electrons in the completed perovskite crystal structure , so there is never any Jahn-Teller distortion in it . The strong Fe–O covalent bonds with their rectilinear octahedral hybrid orbital maintain the cubic symmetry of the structure over a wide temperature range . In the ionic LaNiO3 and LaFeO3 the component ions can shift about in the structure , settling into the most comfortable positions they can . This results in some distortion of their perovskite structures away from the ideal cubic symmetry . My picture VB sketches show covalent bonding in a crystal structure , but they don't differentiate between a purely ionic bonding in it or coordinate covalent bonding , which are similarly portrayed .

A complicating factor is that SrFeO3 itself is an antiferromagnet with TN = 134 K . However , it has an unusual “screw [helical] spin” magnetic structure (Oda and co-workers , their Fig. 4 , p. C1-122) that might not interefere with the metallic bond free electrons , although it could influence the antiferromagnetic induction from the LaFeO3 .

The question will arise with respect to the use of SrFeO3 with LaFeO3 in epitaxial composites : won't there be a mixed-valent resonance between the Fe(IV) 3d4 electrons in the former compound and the Fe3+ cations' 3d5 electrons in the latter component ? I think that's what makes the SrFeO3LaFeO3 combination so interesting . Assuming that there is a negligible diffusion of the various atoms in the layers , the Sr2+ and La3+ cations should act as controllers in their respective layers , preventing the 3d4 – 3d5 resonance , which otherwise might degrade the antiferromagnetic ordering in the LaFeO3 layers . On the other hand , Fe(IV) is expected to be a stronger oxidizer than Fe3+ (I don't have the former's E0red immediately at hand) , so it may tug on the 3d5 electrons anyway , pulling them out of the LaFeO3 layers , thereby creating electrical polarization in them . The SrFeO3LaFeO3 epitaxial film composite would be an very interesting material to prepare and study !

There are three caveats to all these layering experiments : first , the metallic and antiferromagnetic components must fit reasonably well together , and apparently matching the two different layers in any epitaxial film composite is often somewhat challenging . Mismatches in the atomic sizes and bond lengths can produce significant strains in the layer interface , which can degrade the physical properties of the material . For example , as mentioned above with LaFeO3 , the Nel temperature of the antiferromagnet will be noticeably reduced when it is layered onto a typical inert substrate such as SrTiO3 :

“The authors also measured the AFM domain contrast to obtain a Nel temperature for the thin films (TN = 670 10 K) lower than that of bulk LaFeO3 (TN = 740 K). The observed reduction in TN was attributed to epitaxial strain caused by the lattice mismatch” (Grepstad and co-workers , p. 109) .

The second caveat concerns the redox properties of the two different layers : they must be compatible , so for example an oxidizing antiferromagnetic layer doesn't deplete the free electrons of the metallic layer , as I mentioned above concerning the Ni3+ cations in black NiO . The standard reduction potential of the Fe3+ cations in LaFeO3 is 0.771 V , which indicates it's mildly oxidizing . Similarly , copper(II) is also mildly oxidizing (E0red = 0.153 V to Cu1+ ; 0.3419 V to Cu0) , which should be kept in mind if antiferromagnetic La2CuO4 (TN ~ 315 K) is used in layering experiments . On the other hand , Ni2+ cations are “low energy” in redox terms , so NiO (TN ~ 525 K) and La2NiO4 (TN = 320330 K) could be used as induction agents without concern about their redox nature .

The third caveat is a reminder that , although a solid state compound may be antiferromagnetic , it might still be unsuitable as an induction agent because the spins of its singlet electrons have the wrong three dimensional ordering in the crystal , perhaps due to canting (a potential problem with LaFeO3–SrTiO3 composites , as noted above) , or possibly caused by some other unusual arrangement of the spins . For example , the multiferroic antiferromagnet BiFeO3 has an excellent TN = 673 K , but its spins have a peculiar helical arrangement in the crystal . This may or may not be a problem if it's layered epitaxially with a metallic compound ; the spin helix may be straightened out in the layering process .

Another example is iron(III) oxide , a-Fe2O3 , whose TN = 948 K (CRC Handbook of Chemistry and Physics , 87th edition , 2006 , p. 12-107) . At first glance this seems pretty spectacular , but on closer examination its magnetic structure reveals a complicated ordering of the iron 3d5 spins . a-Fe2O3 (with the corundum crystal structure) is indeed antiferromagnetic up to its Morin transition temperature , 260 K , at which point the spin orientation switches from being perpendicular to the c crystal axis to being parallel to it above that temperature . a-Fe2O3 is described as either a canted antiferromagnet or as a ferromagnet from 260 K to 948 K , I suppose depending on which plane of atoms is being considered . It would be difficult to predict in advance how a-Fe2O3 might function as an antiferromagnetic induction agent in epitaxial film composites . It would seem to be one of those “try it and see what happens” situations .

A third interesting example is that of iron monostannide , FeSn . It's a fairly good metallic solid a true metal with a high conductivity of around 2.9 million ohm-1cm-1 at 4.2 K in liquid helium , and it's also antiferromagnetic (TN = 373 K , CRC Handbook of Chemistry and Physics) , but it never becomes superconducting . Why not ? Its reported magnetic susceptibility reflects the alternating layer-by-layer spins of its singlet electrons ; but a subtler , more profound analytical technique , neutron diffraction , clearly shows that within each layer of iron and tin atoms in the crystal , the electron spins are in a parallel (ferromagnetic) orientation . However , the metallic bond XO – the iron atoms' 4p p XO – is also over the same planes of atoms , and the ferromagnetic spin orientation of the free electrons prevents FeSn from ever becoming a superconductor at any temperature . I've discussed the electronic structure of FeSn , and those of FeSn2 , iron , and tin in another Chemexplore web page , “An Appreciation of the Classic Valence Bond Theory”.

The lesson learned here is that each candidate component of the layering experiments , both antiferromagnetic and metallic , must be carefully studied and considered ; its physical and chemical properties must be understood and taken into account . In particular , its crystal , electronic , and magnetic structures must either be well known and understood , or determined , and again carefully studied . With this preliminary information in hand preparations can confidently proceed for its use in the synthesis of an epitaxial film composite with the corresponding second component .

The following Table lists various materials which could be investigated as the antiferromagnetic induction agent and as the metallic substrate in epitaxial film composites intended as high temperature superconductor (HTS) candidates . All these compounds (except cubic SnP) were discussed above or in the Antiferro web page , and are summarized here for the reader's convenience :

The permutations and combinations of these components could provide a veritable cornucopia of research work for solid state scientists , all the while keeping in mind the three caveats to the layering experiments mentioned above .

Having said all that about epitaxial film composites , I still believe that the most important HTS materials must be produced in bulk form (pellets , powders , granules , etc.) , rather than as films . The principal future technological applications of HTS products will be in power transmission , transportation (levitation) , and in MRI machines for hospitals . I suspect that HTS films won't be able to support the powerful magnetic fields (Hc1) and high electrical current densities (Jc) required by these sorts of end-uses ; rather , they would mainly be used in the electronics industry , for example in HTS (or even RTS , room temperature superconductor) printed circuits and chips for supercomputers .

The two important benefits of epitaxial film composite research , as outlined in this web page , will be (assuming such research is undertaken and proves successful) , first , it will support and provide credibility to the antiferromagnetic induction theory , as discussed in the Antiferro web page ; and second , any HTS composites synthesized could be a template from which a bulk version of the material can hopefully be reproduced later by ingenious chemists . Thus , the “film HTS” researchers will be the explorers and pioneers for the “bulk HTS” researchers and developers who follow them . As mentioned above , I believe that transition temperatures for some of the layered composites could reasonably reach 200+ K , and indeed , these sorts of novel materials might be our only hope for ever obtaining a functional RTS . As potentially very valuable and economically important materials – not to mention as fascinating new theoretical systems – they deserve the full attention and careful study of condensed matter physicists and solid state chemists in future research programs .

 

References , Notes , and Further Reading

 

bronzes : P.G. Dickens and M.S. Whittingham , “The Tungsten Bronzes and Related Compounds”, Quart. Rev. 22 (1) , pp. 30-44 (1968) ; H.R. Shanks , P.H. Sidles , and G.C. Danielson , “Electrical Properties of the Tungsten Bronzes”, Ch. 22 , pp. 237-245 in Nonstoichiometric Compounds , R. Ward (ed.) , Adv. Chem. Series 39 , American Chemical Society , Washington , D.C. 1963 ; C.T. Hauck , A. Wold , and E. Banks , “Sodium Tungsten Bronzes”, pp. 153-158 in Inorg. Synth. 12 , R.W. Parry (ed.) , McGraw-Hill , New York , 1970 [republished by R.E. Krieger , Huntington , NY] ; M.J. Sienko , “Electric and Magnetic Properties of the Tungsten and Vanadium Bronzes”, Ch. 21 , pp. 224-236 in Nonstoichiometric Compounds (see above) ; A. Kleiman , “Tungsten Bronzes : Electrochromic Properties and Metal Insulator Transitions”, 22 pp. (November , 2005) ; (PDF , 672 KB) .

shake-'n-bake : This is the preparative technique in ceramic chemistry where stoichiometric quantities of reagents , usually the oxides or carbonates (sometimes hydroxides and oxalates) of the desired elements , are combined together to form complex compounds . The materials are generally quite refractory , and require repeated cycles of grinding together , pelletizing , heating in a furnace , and cooldown . YBCO and related copper oxide superconductors are often prepared this way . A detailed “recipe” and procedure for making YBCO can be found on the web page , “Making High-Temperature Superconductors”, by John Wiltbank , and on this web page , “Preparation , Structure and Properties of a High-Temperature Supeconductor”, by M.S. Whittingham , Institute for Materials Research , State University of New York at Binghampton .

electronic structure of rhenium trioxide : A band structureDOS diagram for rhenium trioxide is presented by : P.A. Cox , Transition Metal Oxides , An Introduction to Their Electronic Structure and Properties , Clarendon Press , Oxford (UK) , 1995 ; Fig. 5.1 , p. 205 . In it , the oxygen 2p bands are entirely below EF , plus a small portion of the Re 5d t2g bands ; the rest of the 5d bands are above EF . In my picture VB analysis , all of the 5d orbitals (plus the 6s orbital) are used for the strong , low energy Re–O covalent bonds , with the 5p orbitals used for the higher energy metallic bond XO .

it does : H.K. Bowen , “Ceramics as Electrical Materials”, pp. 290-314 in the Kirk-Othmer Encyclopedia of Chemical Technology , Vol. 5 , M. Grayson and D. Eckroth (eds.) , John Wiley , New York (1979) ; Fig. 5 , p. 299 .

Pauli paramagnetism : J.D. Greiner and H.R. Shanks , “The Magnetic Susceptibility of Rhenium Trioxide”, J. Solid State Chem. 5 (2) , pp. 262-265 (1972) .

mixed-valent compound : M.B. Robin and P. Day , “Mixed Valence Chemistry – A Survey and Classification”, Adv. Inorg. Chem. Radiochem. 10 , pp. 247-422 , H.J. Emelus and A.G. Sharpe (eds.) , Academic Press , New York , 1967 ; see also : P. Day , “Mixed Valence Chemistry and Metal Chain Compounds”, pp. 191-214 in  Mixed-Valence Compounds : Theory and Applications in Chemistry , Physics , Geology , and Biology , D.B. Brown (ed.) , NATO Advanced Study Institute , Series C , Mathematical and Physical Sciences Series no. 58 , Reidel-Holland (Kluwer Academic Publications , Hingham , MA) , 1980 ; P. Day , “Les Composs Valence Mixte”, La Recherche 12 (120) , pp. 304-311 (mars 1981) ; A.J. Markwell , “Mixed-Valency Compounds”, Educ. Chem. 25 (1) , pp. 15-17 (January , 1988) . I've discussed the four Robin-Day classes in my web page , “New Solar Cells from Mixed-Valent Compounds”, with various examples of each class , and illustrated with sketches of the examples .

Moore : W.J. Moore , Seven Solid States , An Introduction to the Chemistry and Physics of Solids , W.A. Benjamin , New York , 1967 ; Ch. 5 , “Nickel Oxide”, pp. 133-162 . This is one of my favourite chemistry textbooks , which I highly recommend . If your Science Library doesn't have it , you can buy an inexpensive second-hand copy from ABE , the Advanced Book Exchange .

electron superexchange : Cox (op. cit.) : electron superexchange is discussed on pp. 148-153 , and is illustrated in Figs. 3.21 (p. 150) , 3.22 (p. 151) , and 3.33 (p. 152) . See also A. Tressaud and J.M. Dance , Ferrimagnetic Fluorides, Adv. Inorg. Chem. Radiochem. 20 , pp. 133-188 ; H.J. Emelus and A.G. Sharpe (eds.) , Academic Press , New York , 1977 ; electron superexchange is discussed on pp. 136-142 , and is illustrated in Fig. 1 , p. 137 . Moore (immediately above) presents a sketch of the electron superexchange in NiO in his Fig. 5.2(b) , p. 136 .

redox reactions : All the redox half-reactions and their potentials cited in this web page were obtained from either of two sources : D.R. Lide (ed.) , CRC Handbook of Chemistry and Physics , 87th edition , 2006 ; P. Vanysek (ed.) , “Electrochemical Series”, pp. 8-20 to 8-25 ; or A.J. Bard (ed.) , Encyclopedia of Electrochemistry of the Elements , various volumes , Marcel Dekker , New York , ca. 1973-1986 . Note that they were derived either from measurements in aqueous media or from thermodynamic calculations , so we should be cautious when applying them in a solid state environment .

the reaction might commence : Or it might not . Redox equations are thermodynamic in nature ; they describe the energetics of a reaction , but not its kinetics , which may be extremely fast and exoergic (an explosion !) , or so slow as to be unnoticeable . A slow reaction may even require a catalyst to start it or speed it up . Generally , the more finely divided and more thoroughly mixed the reactants are , the faster and more complete the reaction will be . If the reaction Re2O7 + Ni0 + NiO ----------> 2 [ReO3–NiO] is attempted as I've described it above , the researcher should take all the necessary safety precautions : no more than a gram or so of reactants should be combined (and in any case rhenium and all of its compounds are very costly) at one time , at least until safe and successful reaction conditions have been first established on a semi-micro scale ; and safety equipment items , especially eye protection (safety glasses , goggles , safety shield) have been deployed . Also , extremely finely divided metal particles (such as the nickel metal powder suggested) have a tendency to be pyrophoric , so the reagent container and its contents should be safely handled (dispensing , weighing , mixing) in a glove box/bag under nitrogen or argon .

tetrakis(triethylphosphite)nickel(0) : M. Meier et al. , “Tetrakis(Triethyl Phosphite) Nickel (0) , Palladium (0) , and Platinum (0) Complexes”, Inorg. Synth. 13 , pp. 112-117 ; F.A. Cotton (ed.) , McGraw-Hill , New York , 1972 . I was unable to find a commercial supplier for this chemical . The analogous reagent tetrakis(triphenylphosphite)nickel(0) , which would probably be satisfactory for use in the proposed synthesis of the 2 : 1 adduct (ReO3)2–NiO , is offered by Acros Organics .

Tetracarbonylnickel(0) , Ni(CO)4 , might also be tried in the reaction ; it's commercially available and is readily soluble in most organic solvents . However , it's a very toxic , volatile (b.p. 43 C) , thermally unstable (explodes at ~ 60 C) , and pyrophoric chemical . I'm reluctant to recommend this very hazardous compound for investigation in the preparation of (ReO3)2–NiO . Only experienced chemists with an excellent laboratory technique , deploying the appropriate safety measures, and with an efficient fume hood ventilation , should carry out chemical reactions with nickel carbonyl !

acicular needles : J.M. Sugihara and S.R. Newman , “Recrystallization of Organic Compounds From Detergent–Water Systems”, J. Org. Chem. 21 (12) , pp. 1445-1447 (1956) .

now recognized : W.W. Schulz and R.M. Wentzcovitch , “Electronic Band Structure and Bonding in Nb3O3”, Phys. Rev. B 48 (23) , pp. 16986-16991 (1993) ; Fig. 1 , p. 16986 .

magnetic study of NbO : H.R. Khan et al. , “Magnetic and Superconductivity Properties of Niobium Oxides”, Mater. Res. Bull. 9 (9) , pp. 1129-1135 (1974) .

arc furnace : T.B. Reed and E.R. Pollard , “Niobium Monoxide”, Inorg. Synth. 14 , pp. 131-134 , A. Wold and J.K. Ruff (eds.) , McGraw-Hill , New York , 1973 ; reprinted in Inorg. Synth. 30 , Nonmolecular Solids , pp.108-110 , D.W. Murphy and L.V. Interrante (eds.) , John Wiley , New York , 1995 . An excellent review of the arc furnace method of inorganic syntheses with , and preparing , refractory materials : T.B. Reed , “Arc Techniques for Materials Research”, Mater. Res. Bull. 2 (3) , pp. 349-367 (1967) .Theodore Gray describes a home-made arc furnace in “Melting the Unmeltable”, Popular Science , May , 2004 ; available online here . The NbO–NiO mixture might also be fused simply by arc-welding it (a knowledgeable arc welder from a local metal fabrication shop might be employed for this) in an argon atmosphere .

Metallic perovskites : Bowen (op. cit. , it does) , Fig. 11 , p. 307 . I've discussed the design and synthesis of a variety of perovskites with fluoride anions and mixed-valent A cations in another Chemexplore web page , “New Solar Cells from Mixed-Valent Compounds”. These materials should be metallic with appreciable ambient electrical conductivities , and they might exhibit a detectable photovoltaic effect , which would make them interesting candidates as novel solar cells . They might also be examined in this present context , as the metallic substrates in spintronic superconductor layered composites .

barium plumbate : R.D. Shannon and P.E. Biersted , “Single-Crystal Growth and Electrical Properties of BaPbO3”, J. Amer. Ceram. Soc. 53 (11) , pp. 635-636 (1970) ; electrical conductivity data are presented in Fig. 1 and Table II , p. 636 .

Ellis et al. : A.B. Ellis et al. , Teaching General Chemistry , A Materials Science Companion , American Chemical Society , Washington , D.C. , 1993 ; pp. 146 .

Kodama and co-workers : Y. Kodama , N. Murayama , Y. Torii , and M. Yasukawa , “Chemical Preparation and Properties of Semi-Metal BaPbO3 Ceramics”, J. Mater. Sci. Lett. 17 (23) , pp. 1999-2001 (1998) .

Lee and co-workers : T.G. Lee et al. , “Synthesis of YBa2Cu3O7-d–BaPbO3 Composites”, J. Korean Phys. Soc. 31 (3) , pp. 410-413 (1997) [PDF , 2878 KB] . See also : M. Tanaka et al. , “Electrical and Structural Properties of CuO–SrPbO3–Ag Ceramic Composites”, Mater. Lett. 20 (5-6) , pp. 275-278 (1994) .

chemie douce : F.J. DiSalvo , “Solid State Chemstry”, Solid State Commun. 102 (2-3) , pp. 79-85 (1997) ; see pp. 81-82 ; A. Stein , S.W. Keller , and T.E. Mallouk , “Turning Down the Heat : Design and Mechanism in Solid-State Synthesis”, Science 259 (5101) , pp. 1558-1564 (1993) ; C.N.R. Rao , Chemical Approaches to the Synthesis of Inorganic Materials , Wiley Eastern Ltd. , New Delhi , India , 1994 ; pp. 11-13 (and elsewhere in this textbook) ; C.N.R. Rao and J. Gopalakrishnan , New Directions in Solid State Chemistry , 2nd edition , p. 128-137 , Cambridge University Press , Cambridge (UK) , 1997 ; Ohio State web page at http://www.chemistry.ohio-state.edu/~woodward/ch754/syn_cd.htm .

Arjomand and Machin : M. Arjomand and D. J. Machin , “Oxide Chemistry . Part II . Ternary Oxides Containing Copper in Oxidation States-I , -II , -III , and -IV”, J. Chem. Soc. Dalton Trans. 1975 (11) , pp. 1061-1066 ; Table 3 , p. 1062 . Arjomand and Machin claimed to have synthesized pure CaCuO2 , but later research showed that it cannot be produced in bulk by the usual shake-n'-bake technique . By doping up to 0.14 mole fraction of Sr2+ into the lattice of CaCuO2 , the compound Ca0.86Sr0.14CuO2 can indeed be made in a bulk preparation under ambient pressure : T. Siegrist , S.M. Zahurak , D.W. Murphy , and R.S. Roth , “The Parent Structure of the Layered High-Temperature Superconductors”, Nature 334 (6179) , pp. 231-232 (1988) .

chemical vapour deposition : A brief review of CVD techniques is provided by U. Schubert and N. Hsing , Synthesis of Inorganic Materials , Wiley-VCH , Weinheim , Germany , 2000 ; pp. 71-112 , especially Table 3-2 , p. 84 ; several general references for molecular beam epitaxy (MBE) : Wikipedia , “Molecular Beam Epitaxy”, here ; Alex Anselm , “An Introduction to MBE Growth”, here ; I. Amato , Stuff , The Materials the World is Made Of , Avon Books , New York , 1997 ; see especially pp. 119-125 for a description of the molecular beam epitaxy (MBE) technique , including a photograph of the industrial apparatus used for it (Figure 21 , p. 125) . The Condensed Matter Physics Group in the School of Physics & Astronomy at the University of Leeds , UK , operates an MBE apparatus and three sputtering machines in their laboratories . They provide an excellent description of these techniques , as well as photos of the equipment . See their web page , “Thin Film Deposition”, here .

thin film component : R.-A. Barb , R.-B. Mos , T. Petrisor Jr. , and C. Samoila , “LNO Bottom Electrodes for Spintronics on Si and Al2O3 Substrates Prepared by Spin Coating Technique Using Metal-Organic Precursors”, Metal 2010 , Roznov pod Radhostem , Czech Republic , EU (PDF , 347 KB) ; V. Lisauskas , B. Vengalis , K. Sliuziene , and V. Pyragas , “Epitaxial Growth and Oxygen Nonstoichiometry of Magnetron–Sputtered Conductive LaNiO3-D Thin Films”, Phys. Chem. of Solid State 9 (2) , pp. 350-352 (2008) [PDF , 649 KB] ; H. Seim et al. , “Deposition of LaNiO3 Thin Films in an Atomic Layer Epitaxy Reactor”, J. Mater. Chem. 7 (3) , pp. 449-454 (1997) ; J.D. Klein , A. Yen , and S.L. Clausen , “Epitaxial LaNiO3 Interlayers for Ferroelectric Memory Structures”, in Epitaxial Oxide Thin Films and Heterostructures , David K. Fork , Julia M. Phillips , R. Ramesh , and Ronald M. Wolf (eds.) , Mater. Res. Soc. Symp. Proc. Vol. 341 , Symposium F , Pittsburgh , PA , 1994 [PDF , 343 KB] ; Y. Zhu et al. , “Preparation and Conducting Performance of LaNiO3 Thin Film on Si Substrate”, Thin Solid Films 471 (1-2) , pp. 48-52 (2005) ; P.-Y. Lai and J.-S. Chen , “Preparation of Perovskite Conductive LaNiO3 Films by Sol-Gel Techniques”, in Science and Technology of Nonvolatile Memories , O. Auciello , J. Van Houdt , R. Carter , and S. Hong (eds.) ,  Mater. Res. Soc. Symp. Proc. 933E , Symposium G , paper no. 0933-G05-05 , Warrendale , PA , 2006 .

Obayashi and Kudo : H. Obayashi and T. Kudo , “Some Crystallographic , Electric and Thermochemical Properties of the Perovskite-Type La1-xMxNiO3 (M : Ca , Sr and Ba)”, Jpn. J. Appl. Phys. 14 (3) , pp. 330-335 (1975) .

Zhang and co-workers : X.D. Zhang et al. , “Investigation of Room Temperature Electrical Resistivities of LaNiO3-d Thin Films Deposited by RF Magnetron Sputtering and High Oxygen-Pressure Processing”, J. Vacuum Sci. & Technol. A , Vacuum , Surfaces , & Films 24 (4) , pp. 914-918 (2006) .

Sagoi and co-workers : M. Sagoi , T. Kinno , J. Yoshida , and K. Mizushima , “Magnetic Properties of LaNiO3 Films and Josephson Characteristics of Y1Ba2Cu3O7-y/LaNiO3/Au/Ag/Pb Junctions”, Appl. Phys. Lett. 62 (15) , pp. 1833-1835 (1993) .

Lning and co-workers : J. Lning et al. , “Determination of the Antiferromagnetic Spin Axis in Epitaxial Films by X-ray Magnetic Linear Dichroism Spectroscopy”, Phys. Rev. B 67 (21) , pp. 214433 , 1-14 (2003) [PDF , 1398 KB] . These researchers state , on the one hand , that LaFeO3 has an orthorhombic symmetry with a = 5.557 , b = 5.565 , and c = 7.854 ; but then they say ,

“SrTiO3 [a cubic symmetry perovskite] is an ideal substrate for epitaxial growth LaFeO3 because of its well-matching lattice constant of a = 3.905 ” (p. 214433-1) . Well-matching ?

Grepstad and co-workers : J. Grepstad et al. , “Effects of Thermal Annealing in Oxygen on the Antiferromagnetic Order and Domain Structure of Epitaxial LaFeO3 Thin Films”, Thin Solid Films 486 (1-2) , pp. 108-112 (2005) [PDF , 653 KB] .

Ahmed and El-Dek : M.A. Ahmed and S.I. El-Dek , “Extraordinary Role of Ca2+ Ions on the Magnetization of LaFeO3 Orthoferrite”, Mater. Sci. Eng. B 128 (1-3) , pp. 30-33 (2006) [PDF , 238 KB] ; refer to their Fig. 4 , p. 32 .

Peterlin-Neumaier and Steichele : T. Peterlin-Neumaier and E. Steichele , “Antiferromagnetic Structure of LaFeO3 from High Resolution TOF Neutron Diffraction”, J. Magnetism & Mag. Mater. 59 (3-4) , pp. 351-356 (1986) . These researchers noted .......“the extremely small orthorhombic distortion (b/a = 1.002) .......”, in their sample of LaFeO3 .

Koehler and Wollan : W.C. Koehler and E.O. Wollan , “Neutron-Diffraction Study of the Magnetic Properties of Perovskite-like Compounds LaBO3”, J. Phys. Chem. Solids 2 (2) , pp. 100-106 (1957) .

Jacobson : A.J. Jacobson , “New Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells”, PDF (1377 KB) ; see Fig. 7 , p. 9 .

exchange bias : A.E. Berkowitz and K. Takano , “Exchange Anisotropy – A Review”, J. Magnetism & Mag. Mater. 200 (3-4) , pp. 552-570 (1999) [PDF , 434 KB] ; J.-I. Hong , T. Leo , D.J. Smith , and A.E. Berkowitz , “Enhancing Exchange Bias with Diluted Antiferromagnets”, Phys. Rev. Lett. 96 (11) , pp. 117204 , 1-4 (2006) [PDF , 785 KB] .

interesting experiments : F. Nolting et al. , “Direct Observation of the Alignment of Ferromagnetic Spins by Antiferromagnetic Spins”, Nature 405 (6788) , pp. 767-769 (2000) [PDF , 199 KB] ; the web page , “PEEM2 Reveals Spin Alignment in Magnetic Layers” ; A. Scholl et al. , “Exploring the Microscopic Origin of Exchange Bias with Photoelectron Emission Microscopy (invited)”, J. Appl. Phys. 89 (11) , pp. 7266-7268 (2001) [PDF , 424 KB] ; F. Nolting et al. , “Observation of Antiferromagnetic Domains in Epitaxial Thin Films”, [PDF , 568 KB] ; A. Scholl et al. , “Studies of the Magnetic Structure at the Ferromagnet–Antiferromagnet Interface”, J. Synchrotron Rad. 8 (Pt. 2) , pp. 101-104 (2001) [preprint version , PDF , 778 KB] .

Logvenov and co-workers : G. Logvenov et al. , “Comprehensive Study of High-Tc Interface Superconductivity”, J. Phys. Chem. Solids 71 (8) , pp. 1098-1104 (2010) [preprint version , PDF , 1075 KB] ; G. Logvenov and I. Bozovic , “Artificial Superlattices Grown by MBE : Could We Design Novel Superconductors ? ”, Physica C : Superconductivity 468 (2) , pp. 100-104 (2008) [preprint version , PDF , 525 KB] .

W.A. Little : W.A. Little , “Possibility of Synthesizing an Organic Superconductor”, Phys. Rev. 134 (6A) , pp. A1416-A1424 (1964) ; ibid. , “Superconductivity at Room Temperature”, Scientific American 212 (2) , pp. 21-27 (February 1965) ; ibid. , “The Exciton Mechanism in Superconductivity”, pp. 17-26 in W.A. Little (ed.) , Proceedings of the International Conference on Organic Superconductors , J. Polymer Sci. , Part C , Polymer Symposia 29 , Interscience , New York , 1970 .

pellets : S. Srinath , M.M. Kumar , M.L. Post , and H. Srikanth , “Magnetization and Magnetoresistance in Insulating Phases of SrFeO3-d”, PDF , 213 KB .

single crystals : A. Lebon et al. , “Magnetism , Charge Order , and Giant Magnetoresistance in SrFeO3-d Single Crystals”, Phys. Rev. Lett. 92 (3) , pp. 037202 , 1-4 (2004) [PDF , 340 KB] .

epitaxial films : C. Sols et al. , “Microstructure and High Temperature Transport Properties of High Quality Epitaxial SrFeO3-d Films”, Solid State Ionics 179 (35-36) , pp. 1996-1999 (2008) [PDF , 740 KB] .

this analysis : A band theory (molecular orbital theory) analysis of the electronic structure of SrFeO3 is provided by : P.M. Woodward , “Transition Metal Perovskites : Band Structure , Electrical and Magnetic Properties”, Chemistry 754 , Solid State Chemistry Lecture no. 24 (May 27, 2003) [Microsoft PowerPoint slide show , PPT , 666 KB] . SrFeO3 is described on slide 13 as “metallic , spiral AFM , TN ~ 130 K”, with Fe(IV) t2g3 s*1 . Its band structure is more fully described on slide 14 . In slide 9 Woodward also presents a band theory-DOS electronic structure of ReO3 , which is quite similar to that in Cox's textbook (op. cit. , electronic structure of rhenium trioxide) .

Oda and co-workers : H. Oda et al. , “Screw Spin Structure in SrFeO3”, J. de Physique , Colloque C1 , 38 (4) , pp. C1-121 to C1-123 (1977) [PDF , 156 KB] .

 

 

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