Ilmenites as High Temperature Superconductors


The antiferromagnetic (AFM) induction model of high temperature superconductors was introduced and discussed in several previous Chemexplore web pages (the references are presented at the end of the text , below) . AFM induction is an enabling mechanism , a physical effect promoting the association of electron pairs at higher temperatures . The fundamental mechanism of the formation of Cooper pairs is the magnetic force between antiparallel spin electrons [sketch in the Chemexplore logo above] ; this attractive force was calculated to be about 1200 times the strength of the repulsive electrostatic force between the electron pairs .

I have stated elsewhere that All superconductors are metallic solids , but not all metallic solids (in fact , relatively few of them) are superconductors. That is , superconductivity , like ordinary electrical conductivity , is a feature unique to metallic solids , more specifically to the metallic bond in those materials . Two fundamental physical phenomena usually inhibit the appearance of superconductivity in a typical metallic solid . First , almost all of them exhibit Pauli paramagnetism , which is caused by the spins of the unpaired singlet electrons above the Fermi level , EF , in their metallic bond . These free electron spins must be organized into a regular pattern of alternating up-and-down spin orientations in order to form Cooper pairs . Such a spin pattern , when it occurs in a paramagnetic crystalline solid , is an antiferromagnetic spin rgime . The metallic solid can become superconducting only if its Pauli paramagnetism is replaced by an AFM spin pattern , impressed on its mobile , free electrons in the metallic bond above EF . That's what AFM induction is intended to accomplish , as I'll outline in a moment .

The second inhibitory physical phenomenon is the Fermi-Dirac distribution , which organizes the metallic bond free electrons , typically spin-pairing about 99% of them below EF . At first glance this looks almost beneficial , since we want to spin-pair all of the metallic bond electrons anyway . However , it actually results in the remaining 1% or so of the singlet electrons above EF being statistically scattered about the crystal lattice , mostly with very wide separation distances (coherence lengths) . Because of these long distances , the magnetic coupling force which is a function of the inverse square of the coherence length between any given pair of singlet electrons will generally be vanishingly small . No Cooper pairs will be formed , and no superconductivity will appear in the solid .

The Fermi-Dirac distribution is a fundamental property of the metallic bond and is unavoidable . However , we can chemically trick it into accepting electron pairs from non-metal atoms as the spin-paired electrons below EF . The most effective such atoms are undoubtedly those of oxygen , in the form of oxide anions in ionic solids , and oxygen linking atoms in covalently-bonded solids . The oxygen 2 s-p electron pairs can participate in the metallic bond with orbitals (of suitable shape , symmetry , and orientation) of the metal atoms . The low energy 2 s-p oxygen orbitals and their electron pairs form the lower layer , below EF , of the resulting bilayer metallic bond . The metal atom electrons are in the higher energy level above EF . However , now they are located very close together on neighbouring metal atoms , and almost 100% of them are above EF , resulting in a very rich population of mobile , free electrons with very short coherence lengths ........ ideal for superconductivity ! So , while we can never avoid the Fermi-Dirac distribution , we can chemically satisfy it with the the inert oxygen filler electrons below EF , and leaving the all-important metal atom electrons above EF to form the Cooper pairs .

I have discussed the bilayer metallic bond at some length in several other Chemexplore web pages . A splendid example of it is found in rhenium trioxide , ReO3 , whose suggested electronic structure , illustrating its bilayer metallic bond , is shown in a Valence Bond sketch [GIF image , 34 KB ; underlined blue hyperlinks can be clicked when online to retrieve the article cited . The requested document will open in a new window] . The electrical conductivity of ReO3 is 144,300 ohm-1cm1 at 293 K , almost 2 times that of its parent element , rhenium (58,140 ohm-1cm1 at 273 K ) . Obviously , the oxygen linking atoms are providing their 2 s-p electron pairs to the Re–O metallic bond , “bulking it up” below EF and pushing most of the rhenium 5d1 valence electrons above EF . Despite its superb electrical conductivity – comparable or even superior to that of many metallurgical metals – rhenium trioxide never becomes superconducting , even very close to Absolute Zero . However , I've suggested in another Chemexplore web page that if ReO3 can be layered with a strong AFM induction agent (my favourite is nickel oxide , NiO) , its Pauli paramagnetism would be reordered into an AFM spin rgime , and it could then become an excellent HTS – high temperature superconductor – composite material .

This web page will principally address chemical aspects of AFM induction , and will assume in all cases that a successful bilayer metallic bond can be formed in the proposed new compounds .


AFM Induction in Heterostructures


In a classic High School physics demonstration , an iron nail is stroked with a magnet until it is shown to have become magnetic itself , for example by picking up small iron or steel objects like paper clips . The individual iron atoms are ferromagnetic , but their magnetic moments are ordinarily (thermally) randomized in the bulk metal . The magnet stroking organizes the ferromagnetic spins into macroscopic domains , regions in the metal lattice with cooperative spins whose additive magnetic moments produce a strong , observable external magnetic field about the iron object .

In a similar manner an antiferromagnetic spin rgime in one lattice can be impressed onto the spin rgime of paramagnetic electrons in a neighbouring lattice . An extraordinary experimental demonstration of this was provided by Nolting and co-workers in 2000 . They synthesized a nanoscale heterostructure (sandwich) comprised of four different atomic layers :

“We investigated a thin [1.2 nm , covered with a protective 1 nm Pt cap] Co film on top of a 40 nm LaFeO3 film grown on SrTiO3 (001) . The sample was prepared in a molecular beam epitaxy system with the LaFeO3 film grown using a block-by-block growth method at 750 C under a beam of atomic oxygen and a partial O2 pressure of 5 x 10-6 Torr . This method has been shown to yield high-quality epitaxial films” (p. 767) .

These researchers were able to show that the AFM spin rgime of the LaFeO3 (TN ~ 750 K) was impressed onto the normally ferromagnetic spin rgime of the cobalt metal atoms . They concluded ,

“Our results open the door for investigations of various ferromagnetic-antiferromagnetic systems consisting of single crystal , polycrystalline or even amorphous materials . As such , they may hold the key to a definitive understanding of the ferromagnetic-antiferromagnetic exchange bias phenomenon” (p. 768) .

The valence shell electrons of elementary cobalt are 3d7 4s2 ; ferromagnetism in the bulk metal is produced by the parallel spin orientations of the 3d electrons , while its 4s electrons are delocalized in its metallic bond . Presumably the LaFeO3 also impressed its AFM ordering onto the latter electrons as well , extinguishing their Pauli paramagnetism and reorienting them into an antiparallel spin state . This AFM re-ordered cobalt might now be superconducting , albeit at a very low temperature in the liquid helium range . Obviously , the elementary metallurgical metals don't have any oxygen 2 s-p electron pairs to bulk up their metallic bonds , which are thus monolayer , not bilayer . This remarkable experiment by Nolting and co-workers suggests that all sorts of metallic solids , including the elementary metals , could be layered with AFM induction layers in heterostructures to form a wide variety of new superconducting composites [GIF image (31 KB)] .

So , instead of Nolting's ferromagnetic-antiferromagnetic exchange , we now have Pauli paramagnetic-antiferromagnetic exchange as an enabling mechanism in HTS materials . The following simple sketch outlines the salient features of the AFM induction model :

Very likely all of the presently known HTS compounds utilize this AFM induction . Let's consider three bismuth-based superconductors : PrO0.5F0.5BiS2 , Tc = 2.5–5.5 K (Awana and co-workers , 2012) ; Ba0.6K0.4BiO3 , Tc = 29.8 K (Cava and co-workers , 1988) ; and Bi2Sr2Ca2Cu3O10+x [BSCCO-2212] , Tc ~ 110 K (Maeda and co-workers , 1988) . In the first compound , the metallic bond is likely in the bismuth 7s XO (XO = crystal orbital = conduction band = metallic bond) , extending through the BiS2 layers ; the PrO0.5F0.5 part of the structure is the electronically inert layer with the spectator ions . This compound appears to have a Bi–Bi monolayer metallic bond , and there is evidently little if any AFM induction from the sulfur atoms . The enabling mechanism for superconductivity in PrO0.5F0.5BiS2 is undoubtedly the classic BCS phonon-assisted Cooper pair formation .

In Ba0.6K0.4BiO3 there also is no AFM induction present , but its 7s XO is now a bilayer metallic bond , being bulked up by the oxygen linking atoms's 2 s-p electron pairs (the compound has a cubic perovskite structure) . The XO has a rich population of free , mobile electrons above EF with very short coherence lengths , so when the the material's Pauli paramagnetism is finally extinguished they can readily condense into Cooper pairs .

BSCCO-2212 is a genuine HTS compound , having a bilayer Bi–O metallic bond like Ba0.6K0.4BiO3 and AFM induction from its copper(II) oxide layers . BSCCO-2212 can be thought of as a heterostructure comprised of alternating layers of metallic Sr2Bi2O5+x with mixed-valent Bi(III,V) , and non-metallic but AFM layers of Ca2Cu3O5 , with homovalent copper(II) :

To recap : PrO0.5F0.5BiS2 , monolayer metallic bond , no AFM induction , Tc = 2.5–5.5 K ; Ba0.6K0.4BiO3 , bilayer metallic bond , no AFM induction , Tc = 29.8 K ; and Bi2Sr2Ca2Cu3O10+x , bilayer metallic bond and AFM induction , Tc ~ 110 K . Other HTS compounds resemble BSCCO-2212 in this regard :

The future of genuine HTS materials is unquestionably in the realm of heterostructures , sandwich compounds in which metallic layers alternate with strongly AFM induction layers (my personal definition of “genuine HTS” is that of a superconductor with Tc = 100 K +) . The design and synthesis of such heterostructures is in the domain of solid state chemistry ; thus , in the coming years chemists must assume a creative leadership role in superconductor research and development .


Ilmenites as Naturally-Occurring Heterostructures


Numerous synthetic heterostructures are already known ; as mentioned , BSCCO-2212 can be considered as such , as are all the HTS cuprates such as YBCO . Ferropnictides are heterostructures comprised of alternating metallic-covalent layers such as FeAs (in which the electrical conduction and Cooper pair formation occurs) and ionic layers of spectator ions such as LaO. These two chemically different types of layers are electrostatically bonded together : [LaO]1+ [FeAs]1- . The recently-synthesized compound PrO0.5F0.5BiS2 , noted above , also has alternating metallic-covalent layers (BiS2) and ionic layers (PrO0.5F0.5) .

Many of Nature's crystal structures are also heterostructures . Some of the layered minerals such as mica are readily apparent as heterostructures ; other crystalline materials have less obvious heterostructural forms . Ilmenite , with the empirical formula AMX3 , is one such naturally-occurring multilayered compound .

The mineral ilmenite chemically is iron(II) titanate , FeTiO3 . It was originally discovered near , and named after , Lake Ilmen in Russia . Large crystals of ilmenite are black in appearance and have a noticeable metallic luster :

This photo was copied from the Wikipedia web page , Ilmenite. I thank the author of this picture , and Wikipedia , for implied permission to reproduce it here .

Ilmenite is one of the principal ores of titanium metal and its compounds , of which titanium dioxide is by far the most important . The iron component of ilmenite is also extracted for conversion into iron and steel products [Rio Tinto tour , PDF , 518 KB . Note : this file can be opened only with Adobe Acrobat Reader v. 6 or later . If desired , this application can be downloaded for free from] . The name ilmenite now applies to the family of compounds having the AMX3 empirical formula and general crystal structure shown in the following sketch :

This sketch was copied from the Wikipedia web page , Ilmenite (I sized-reduced it somewhat so it would fit comfortably on this web page) . Again , my thanks to the author of this sketch , and Wikipedia , for implied permission to reproduce it here .

A somewhat simpler sketch of the ilmenite structure was presented by Liu , Hong , and Tian :

(my thanks to the author and/or copyright owner of this illustration)

In the AMX3 formula A is a small metal cation , M is a small covalently-bonded metal atom , and X is a non-metal atom , usually an oxide anion or oxygen linking atom in covalent structures . The A and M layers of atoms are separated by the X anions : A–X–M–X–A–X–M– etc. . A and M are octahedrally coordinated by six X anions or atoms (three in the X layer above and three in the X layer below) . The anions have a distorted tetrahedral coordination by four metal cations or atoms .

In the literature I reviewed on the subject of ilmenites several authors referred to them as a sub-type of the more general family of corundum compounds , which have the empirical formula A2X3 . Ilmenites and corundums have the same crystal structure ; both materials have alternating layers of hexagonally closed-packed X anions , whose interstices are filled with the A cations and M atoms in the sequence A–A–vacancy–A–A–vacancy ...... and similarly M–M–vacancy–M–M–vacancy ...... (as shown in the above sketches) . In corundum compounds A and M are identical .

When the X anions are oxides , the X3 total negative charge is –6 , and the A and M positive charges must add up to +6 . The three combinations of 2+4 , 3+3 , and 1+5 are thus possible for the A + M positive charges . In FeTiO3 the A cation is Fe2+ , while the M atom is Ti(IV) [I use superscript Arabic numerals for ionic cations , and Roman numerals for covalently bonded metal atoms] . In the ilmenite compound lithium niobate , LiNbO3 , A is Li1+ and M is Nb(V) . I can't think of a 3+3 ilmenite offhand ; possibly the compound AlYO3 might have the ilmenite crystal structure . The following tabulation presents a selection of ternary metal oxides known to have the ilmenite crystal structure :

The alternating layers of the two different A and M atoms in ilmenites suggests that this natural heterostructure might be adapted in the search for new candidate compounds for testing the AFM induction model of superconductivity . The A cations could be selected from the Transition metal elements known for their corresponding AFM compounds . The M covalently bonded atoms could form , with oxide anions or oxygen atoms , the metallic layer in the composite material .

Various antiferromagnetic Transition metal compounds are presented in the following sketch :

In the above tabulation I use the terms interlayer and intralayer to refer to the different relative orderings of the magnetic spins . In the first case the spins are all in a parallel order within the same plane in the crystal latttice , but are in opposite directions from plane to plane . In the intralayer case (intra, by analogy with intramolecular, within the same molecule) the spins have an antiparallel ordering from metal cation to metal cation within the same layer , and of course throughout all the layers in the crystal .

The overall magnetic ordering in a compound can usually be readily determined by a study of its magnetic susceptibility over a wide temperature range , often from near Absolute Zero up to room temperature . This can be accomplished with a common laboratory apparatus such as the Gouy balance . However , a more subtle and sophisticated method of directly determining the magnetic structure of a crystalline solid is by neutron diffraction . While chargeless , neutrons have a surrounding magnetic field that results in their deflection by those of the singlet , unpaired electrons in atomic kernels in a crystal lattice . The magnetic structure of the material can then be determined from an analysis of the neutron scattering pattern , somewhat like the crystal structure being determined from an analysis of the X-ray diffraction pattern by the atoms in the lattice .

The intermetallic compound iron monostannide , FeSn , clearly demonstrates the interlayer AFM rgime . FeSn has an overall antiferromagnetic ordering , whose Nel temperature has been reported as TN = 373 K . However , as is typical with hexagonally-packed lattices , its individual layers of iron atoms have ferromagnetic (parallel order) spins , while the layer-by-layer spin order is antiparallel , resulting in the observed AFM description :

FeSn is an excellent electrical conductor : 12,500 to ~ 15,400 ohm-1cm-1 (depending on the crystal axis) at room temperature , to ~ 2.9 million ohm-1cm-1 in liquid helium at 4.2 K . It apparently never becomes superconducting ; the ferromagnetic ordering of its metallic bond (4p pi XO) free electrons very effectively prevents that from ever happening . A similar situation occurs with the metallic compound iron monophosphide , FeP , which also has the hexagonal NiAs crystal structure . FeP is overall AFM , with TN = 123 K . Despite its excellent electrical conductivity (somewhat similar to that of FeSn) , it also never becomes superconducting .

Nickel(II) oxide provides a good example of the intralayer AFM ordering pattern . I've scanned a nice illustration of the magnetic structure of NiO from Moore's textbook , Seven Solid States , into the following sketch (my thanks to the copyright owner) :

Within the individual Ni–O layers the Ni2+ 3d electron spins are in an antiparallel pattern ; this AFM ordering extends throughout the entire lattice of the crystal .

Clearly , the AFM induction layers in any heterostructure intended for HTS must have the intralayer type of AFM ordering ; the undesireable interlayer variety will actually prevent the condensation of any Cooper pairs from the metallic bond free electrons , and so inhibit the appearance of superconductivity in the material . The magnetic structures of the four titanate ilmenites in the above tabulation are of both AFM types : MnTiO3 has the good intralayer pattern , while FeTiO3 , CoTiO3 , and NiTiO3 all have the bad interlayer ordering :

The above sketch of FeTiO3 was copied from the web page , “Magnetic and Structural Properties of Hemo-Ilmenite Solid Solutions”, from the Metal Physics and Technology Laboratory , ETH Zurich , Switzerland . My thanks to its author and/or copyright owner . This sketch , incidently , nicely illustrates the hexagonal atomic packing in the ilmenites and corundums (Fe2O3) .

However , that doesn't necessarily mean that ilmenites with other M metal atoms might behave similarly . At this early stage in the rational design and synthesis of new heterostructures for HTS we simply can't predict in advance the magnetic structures of the materials as AFM compounds . It will have to be an empirical process of trial-and-error until a more extensive database on the subject has been compiled .

The strength of the AFM ordering in a compound , roughly indicated by its Nel temperature , also can't be predicted from first principles at this time . A comparison of the TN values of the divalent Transition metal oxides versus those of their corresponding ilmenite titanates suggests , as a general rule of thumb , that the inclusion of diamagnetic spectator ions or atoms in the T.M. oxides will significantly lower TN in the corresponding ternary oxides . This phenomenon was noted in 1975 by Arjomand and Machin (1) , in their study of the magnetic properties of ternary nickel oxides . For example , NiO has a very high TN = 525 K , while the compound BaNiO2 was only weakly antiferromagnetic . The most strongly AFM ternary nickel compound studied by them , La2NiO4 , was later shown to have a TN ~ 320 K , considerably inferior to that of NiO . The NiO layers in NiTiO3 (TN = 23 K) are chemically and magnetically quite different than those in NiO !

With these considerations in mind , the AFM induction layers in the ilmenite heterostructures in the following sections will be based on the Transition metal divalent oxides . Various A2+M(IV)O3 candidate compounds and their doped , mixed-valent composites will be considered , all the while hoping that the correct form of AFM ordering (intralayer) will be present in the new materials .


Proposed New Ilmenite Heterostructures


In this study the approach taken in designing new ilmenite heterostructures will first be to select a suitable metallic layer , whose tetravalent M metal atoms are octahedrally coordinated by oxygens , and having covalent M–O bonds . Possible M candidates include Ti(IV) , V(V) , Nb(IV) , Ta(IV) , Cr(IV) , Mo(IV) , W(V) , Mn(IV) , Re(IV) , Fe(IV) , Sn(IV) and Pb(IV) . Most of these M(IV) oxides are known to be metallic to a certain extent , as shown in the following graphs of the electrical conductivity of various metal oxides over a wide temperature range (my thanks to the copyright owner of this illustration) :

The derivative ternary oxide should have some sort of metallic behaviour that might be modified by its AFM layer .

The second step wll be to combine the MO2 layers with known AFM binary AO oxides (see the above Table) to obtain the desired AMO3 ilmenite compound . The AFM oxides will include MnO , FeO , CoO , NiO , and CuO . CuO is strongly AFM , with TN = 230 K . It has its own unique crystal structure [GIF image , 38 KB] consisting of planes of interlocking CuO2 ribbons joined at the Cu atoms .

As a third step the A2+M(IV)O3 ilmenites , which should be naturally” metallic , could be further modified by controlled valence doping , for example by the corresponding A1+M5+O3 compound . A1+ is usually the fairly small Li1+ cation (crystal ionic radius = 0.76 , six-coordinate , per Shannon-Prewitt ; CRC Handbook of Chemistry and Physics , 87th edition , 2006 , p. 12-12) . The resulting M(IV)–M(V) mixed-valent compound should also be electrically conductive . The chemically reducing systems based on Ti(IV) and Sn(IV) will exceptionally be doped by Ti(III) and Sn(III) , respectively .

It might be appropriate to begin with the titanate ilmenites , as FeTiO3 is the original and prototype ilmenite compound . However , as noted above , FeTiO3 has the wrong sort of magnetic structure (interlayer) for AFM induction . Its closely related cousin MnTiO3 apparently has a suitable intralayer AFM spin ordering pattern , so we'll choose this material for chemical modification .

The MnO layers in MnTiO3 will be the AFM induction layers in the heterostructure composite , and the TiO3 part of the compound will be converted into the metallic layer . The way to do this is to partially reduce its Ti(IV) to Ti(III) . The compound LiTi2O4 , which has the normal spinel crystal structure , was synthesized by Johnston in 1976 , and was found to be a low temperature superconductor (Tc = 13.7 K) . It has Ti(III)–Ti(IV) : Li1+Ti3+ Ti4+ (O2-)4 , and clearly is a Robin-Day Class II mixed-valent compound and a metallic solid , but it lacks any sort of AFM induction .

The usual solid state chemistry technique to make MnTiO3 mixed-valent would be to dope it with varying mole ratios of a closely related compound containing Ti(III) . The dopant in this case will be A3+Ti3+O3 , where A is an inert trivalent cation . The inert dopant cations will mix with the Mn2+ cations in the MnO layers , so they should be matched as closely as possible with the Mn2+ cations in size , as measured by their crystal ionic radii . The crystal ionic radius of octahedral , six-coordinate Mn2+, per Shannon-Prewitt , is 0.83 . The best trivalent cation match to this is probably In3+ (r = 0.80 ) , so InTiO3 would be the recommended Ti(III) dopant for MnTiO3 .

Liu , Hong , and Tian (family of compounds) mention “complex ilmenites” such as Zn1-x(Mx)TiO3 [M = Mg , Ni , and Co] and ilmenite-type solid solutions , eg. (Zn,Mg)TiO3 , in their report . These latter compounds are sometimes more thermally stable than their simpler parent titanates , which may crystallographically rearrange into more thermodynamically stable perovskites , spinels , and rutiles at high (> 900 C) temperatures :

“For example , ZnTiO3 ilmenite will decompose to rutile [TiO2] and ZnTi2O4 [normal spinel] when the sintering temperature is above 900 C , but the stability region of the ilmenite phase extends from 900 to 1150 C in (Zn,Mg)TiO3 ilmenite-type solid solution” (p. 323) .

The dopant InTiO3 might form such a solid solution , MnxIn1-xTiO3 , with the substrate MnTiO3 .

In the suggested synthesis of In-doped MnTiO3 , MnO and TiO2 could be the precursor reagents for MnTiO3 : MnO + TiO2 -------> MnTiO3 . Similarly , InO + TiO2 -------> InTiO3 . InO is the reducing agent for the Ti(IV) and thus provides the electrons for the metallic bond in the composite . It could be prepared from the reproportionation of indium metal (m.p. 156 C) and indium(III) oxide (subl. ~ 850 C) : 1/3 In0 + 1/3 In2O3 -------> InO . When combined with TiO2 these indium precursor reagents should produce the required dopant : 1/3 In0 + 1/3 In2O3 + TiO2 -------> InTiO3 .

Doped MnTiO3 could be made in a three step procedure : first , prepare a quantity of the pure substrate , MnTiO3 ; second , make a sample of the pure dopant , InTiO3 ; then third , dope the substrate with increasing mole ratios of the dopant : x MnTiO3 + (1-x) InTiO3 -------> MnxIn1-xTiO3 , where x = a mole ratio between 0 and 1 taken experimentally by the researcher . Alternately , the doping might be carried out in a one-pot shake-'n-bake reaction :

x MnO + 1/3(1-x) In0 + 1/3(1-x) In2O3 + TiO2 ----- (heat , inert atm.) -----> MnxIn1-xTiO3 ; x = 0 to 1 .

As the Ti(III)–Ti(IV) system is chemically reducing in nature it must be carefully protected against air oxidation by an inert blanket of pure nitrogen or argon .

The resulting composite MnxIn1-xTiO3 should :

* have the ilmenite crystal structure ;

* be a metallic solid , with the metallic bond in the TiO3 layers , resulting from the ultra-fast resonance of the Ti(III) 3d1 electrons between the Ti(III) and Ti(IV) kernels ;

* have MnO-based AFM induction layers containing redox-inert In3+ dopant cations ; the Nel temperature of the doped composite will likely decline from TN = 65 K of the pure MnTiO3 substrate as increasing mole ratios of In3+ cations are added to it . We have the uncomfortable scenario of the composite becoming more and more metallic as more and more dopant electrons from the indium are added to it ; the AFM layers correspondingly become weaker and weaker magnetically . There might thus be a balance point in the MnxIn1-xTiO3 system at which there is a simultaneous optimum metallic doping and AFM induction strength . As that condition can only be determined experimentally , the researcher would have to synthesize at least a half dozen or so of the doped composites at varying mole ratios x, to discover which has the highest Tc (if any) .

Another possibility is that the InO” would form a separate layer in the structure , having a cationic charge , with an anionic charge on the “TiO” layers : [In3+O2-]1+ [Ti3+O2-]1- , or [In3+O2-]1+ [Ti(IV) O2-]2- + e- , where e- is the metallic bond electron . This would be a more favorable scenario , since the MnO AFM layers wouldn't be degraded by the intrusive In3+ cations . Such an electrostatic bonding of the electron donor layers (InO) with the electron acceptor layers (TiO) would resemble that in the ferropnictides such as LaOFeAs : [La3+O2-]1+ [FeAs]1- (the FeAs has covalent , not ionic , bonding) .

Selection of a very small A cation might ensure this alternate “separate layers” scenario . The Al3+ cation (crystal ionic radius , 6-coordinate octahedral , r = 0.54 ) , which is found in the crystallographically related corundum (Al2O3) structure , could be tried in the mixed-valent composites :

x MnO + 1/3(1-x) Al0 + 1/3(1-x) Al2O3 + TiO2 ----- (heat , inert atm.) -----> MnxAl1-xTiO3 ;

x = 0 to 1 ; alternately (and probably better) ,

x MnO + (1-x) Al2O3 + (1-x) TiO + (1+ x) TiO2 ----- (heat , inert atm.) -----> MnxAl1-xTiO3 .

In the second synthesis titanium(II) oxide is suggested as the reducing agent . TiO is a metallic solid with a shiny , bronze appearance and is appreciably electrically conductive (metal oxides conductivity graph , above) :

(my thanks to the copyright holder of the above photo)

TiO has Ti–Ti covalent bonds , is fairly refractory (m.p. 1770 C) , and is commercially available (eg. Alfa-Aesar , as a –325 mesh powder) at a moderate cost . It could be the reducing agent in the mixture and the source of the metallic bond electrons in the Ti–O layers . TiO would be a milder reducer than aluminum powder , which when combined with the Transition metal oxides results in an extremely energetic thermite (Goldschmidt) reaction , usually with the production of the molten Transition metal .

Tin(III)–tin(IV) is an analogous mixed-valent metallic system that would be interesting to study in this ilmenite context . A possible synthesis of the heterostructure composite MnxIn1-xSnO3 would resemble that of MnxIn1-xTiO3 :

x MnO + 1/3(1-x) In0 + 1/3(1-x) In2O3 + SnO2 ----- (heat , inert atm.) -----> MnxIn1-xSnO3 ;

x = 0 to 1 ; in another variation ,

x MnO + (1-x) Al2O3 + (1-x) Sn0 + (3+x) SnO2 ----- (heat , inert atm.) -----> MnxAl1-xSnO3 .

Tin(III) is a rather rare chemical species . The compound SnP , formally tin(III) phosphide [but really Sn(IV)+(e-)P(III) , with a metallic bond and Sn–P covalent bonds] , was prepared by Donohue in 1969 . It was a metallic solid and an excellent electrical conductor at all temperatures . Its cubic rocksalt form was superconducting near Absolute Zero (Tc = 2.8–4.0 K) . The MnO layers in the composite hopefully would induce some AFM ordering into the former's metallic bond electrons and result in a somewhat elevated Tc in one of the samples , perhaps from 20–30 K or so .

Note that the E0red = – 1.185 V for Mn2+ to Mn0 is fairly high , so there is no possibility of the mild reducing agents Ti(III) and Sn(III) reducing the Mn2+ to manganese metal in the reaction mixture .


Vanadium , Niobium , and Tantalum Ilmenites


The Group 5 (VB) family of Transition metal elements could also be examined as metallic layer components in new ilmenite heterostructures . Vanadium , niobium , and tantalum have three common valence states : 3 , 4 , and 5 , the latter being the empty” one [nd0 (n+1)s0] . As mentioned above , the compound LiNbO3 is a well-known ilmenite ; it might be used as the Nb(V) dopant for a Nb(IV) ilmenite substrate such as A2+Nb4+O3 [A = Mn , Fe , Co , Ni , and Cu] .

Two of the three M(IV) dioxides are well known , stable compounds . VO2 (m.p. 1967 C) has a deep blue color :

“At room temperature VO2 has a distorted rutile structure with shorter distances between pairs of V atoms indicating metalmetal bonding . Above 68 C the structure changes to an undistorted rutile structure and the metalmetal bonds are broken causing an increase in electrical conductivity and magnetic susceptibility as the bonding electrons are “released”. The origin of this metal to insulator transition remains controversial and is of interest in condensed matter physics”.

from the Wikipedia web page , Vanadium(IV) oxide .

Niobium dioxide (m.p. 1915 C) is bluish-black , having a rutile crystal structure and short NbNb distances , possibly metalmetal covalent bonds . Tantalum dioxide exists , but apparently is very little-known . VO2 and NbO2 at least are electronically-active compounds and imply the A2+Nb4+O3 ilmenites should similarly be so . They , and their doped M(IV,V) ilmenite composites , would be very interesting to prepare and study as potential superconductor candidate materials .

The commonest and least expensive reagent of V , Nb , and Ta is in each case the pentoxide , so the following proposed syntheses of ilmenite superconductor candidates will utilize them as starting materials . The formation of A2+M4+O3 from M2O5 requires the combination of “A2O” with it . A2O might be prepared in situ by the combination of A0 and AO , when the latter reagent is readily available at a modest cost (MnO , NiO) . This sort of synthesis should be fairly straightforward :

Mn0 + MnO + V2O5 ----- (heat , inert atm.) -----> MnVO3 ; therefore ,

x Mn0 + x MnO + x V2O5 ----- (heat , inert atm.) -----> x MnVO3 [substrate] .

For the dopant : Li2CO3 + V2O5 ----- (heat , inert atm.) -----> LiVO3 + CO2 (g) ; therefore ,

(1-x) Li2CO3 + (1-x) V2O5 ----- (heat , inert atm.) -----> (1-x) LiVO3 + (1-x) CO2 (g) .

Overall reaction : x Mn0 + x MnO + (1-x) Li2CO3 + V2O5 ------- (heat , flowing nitrogen atmosphere) -------> MnxLi1-xVO3 + (1-x) CO2 (g) .

As usual , x = a mole fraction between 0 and 1 taken experimentally by the researcher . The composite material MnxLi1-xVO3 will have mixed-valent V(IV,V) and should be metallic in nature ; plus , it will have MnO AFM layers , which hopefully will induce superconductivity in it at a modest (~ 20–30 K) temperature . Of course , we have no idea how the compound MnVO3 behaves magnetically , or even if it actually is antiferromagnetic , or if it is AFM , how strong the induction will be in the composite . Design will go only so far ; then we must (still !) rely on serendipity to obtain promising results .

The above reaction sequence could be modified to prepare CoxLi1-xVO3 using the readily available cobalt(II) carbonate , CoCO3 , as the cobalt source .

Doped copper(II) vanadate might be prepared from Cu2O and V2O5 :

x Cu2O + (1-x) Li2CO3 + V2O5 ----- (heat , inert atm.) -----> CuxLi1-xVO3 + (1-x) CO2 (g) .

Simple redox equations suggest that a rapid transfer of electrons from the Cu(I) to the vanadium(V) – a fairly strong oxidizer – should occur :

Actually , V2O5 + 6 H+ + 2e- -------------> 2 VO2+ + 3 H2O ; E0red = 0.957 V ;

More simply : V(V) + e- -------------> V(IV) ; E0red = 0.957 V ;

Combine with : Cu1+ – e- -------------> Cu2+ ; E0ox = – 0.153 V ;

Net reaction : V(V) + Cu1+ -------------> V(IV) + Cu2+ ; E0T = 0.804 V .

The three dopant compounds for this Group 5 series , LiVO3 , LiNbO3 , and LiTaO3 , are all commercially available at a moderate cost (Alfa-Aesar) . They might form stable , “smooth”, solid solutions with the substrate ilmenites M2+VO3 , M2+NbO3 , and M2+TaO3 , respectively [M = Mn , Fe , Co , Ni , and Cu] .

It would probably be better , though , to prepare them in situ in the reaction mixture , rather than to use them as dopants with A2+M4+O3 . Lee and co-workers failed to produce a new compound with alternating Pb–O and Cu–O layers when sintering BaPbO3 and YBCO together at 900–1000 C . They obtained a complex solid with fused microcrystals of both starting materials :

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

Apparently the best technique is to start with primary materials such as BaCO3 , PbO , CuO , and Y2O3 ; or in our ilmenite case , with the reagents shown in the above chemical equations . When using these simple precursor chemicals the resulting formulas for the heterostructure synthesis can look dauntingly complex , but usually (for example , BSCCO-2212 , sketch above) the multilayer material is successfully produced . Another good example is the thallium-doped mercury cuprate , Hg0.8Tl0.2Ba2Ca2Cu3O8.33 , which emerged resplendent from a witch's brew of simple starting compounds , having the excellent Tc = 138 K , which still holds the record for the highest ambient pressure superconductor transition temperature .

Heterostructure formation is enabled by the different chemical bonding in the various layers . Higher-valent metal cations bonded to oxygen atoms generally have MO covalent bonds , while certain Transition metal cations , notably Cu1+ and Cu2+, form very strong coordinate covalent bonds with oxygen linking atoms . Metal cations (Alkali , Alkaline Earth , Rare Earth) with Inert Gas kernels have ionic bonds to oxide anions . The covalent and ionic layers remain cleanly separated in the heterostructure , with no appreciable mixing of their metal atom components . A simple analogy would be the three-layer liquid heterostructure formed by a layer of mercury metal in a beaker , over which a layer of water is poured ; above the water layer the third , least dense layer of mineral oil is added . The three layers are comprised of completely different chemicals with radically different chemical bonds , and are mutually repellent and immiscible .

An A2O intermediate might also be prepared in situ from the reproportionation of A0 and its A2O3 oxide . This is illustrated for iron(II) oxide , FeO , which is surprisingly expensive (per my Alfa-Aesar catalog) :

2/3x Fe0 + 1/6x Fe2O3 + + (1-x) Li2CO3 + V2O5 ----- (heat , inert atm.) -----> FexLi1-xVO3

+ (1-x) CO2 (g) .

The niobium and tantalum composites , AxLi1-xNbO3 and AxLi1-xTaO3 respectively (M = Mn , Fe , Co , Ni , and Cu) , might similarly be synthesized and studied .


Molybdenum and Tungsten Ilmenites


The Group 6 (VIB) elements , chromium , molybdenum , and tungsten , can have a wide range of valence states : 2 and 3 (ionic) and 4 , 5 , and 6 (covalent) . It should be possible to synthesize their A2+M4+O3 ilmenites and Li1+A2+M4,5+O3 composites , the latter by doping with the corresponding Li1+M5+O3 compounds .

These M(IV) dioxides are well-known metallic solids . Chromium dioxide is a black or brownish-black solid that decomposes at ~ 400 C . It has the rutile crystal structure and is ferromagnetic , and has been used as the magnetic recording medium in compact cassettes . It is an excellent electrical conductor at all temperatures . I discussed the properties and electronic structure of CrO2 in other Chemexplore web pages (Solids and Chromium Dioxide) . Molybdenum dioxide is brownish-violet and decomposes at ~ 1100 C . It is described as a metallic conductor , with a distorted rutile structure and possibly having MoMo covalent bonds . Tungsten dioxide is somewhat similar to MoO2 . Wikipedia comments about WO2 :

“The bronze-colored solid crystallizes in a monoclinic cell . The rutile-like structure features distorted octahedral WO6 centers with alternate short W–W bonds (248 pm) . Each tungsten center has the d2 configuration, which gives the material a high electrical conductivity”.

These MO2 compounds are reminiscent of the chromium , molybdenum , and tungsten bronzes , which are also highly colored metallic solids with a substantial electrical conductivity (mostly , at high doping levels) . Even the undoped Group 6 A2+M4+O3 ilmenites might be quite metallic and have a respectable superconductor Tc .

The MO3 oxides , which are the most common and readily available of the Group 6 compounds , might be used in the synthesis of their corresponding ilmenites , simply by combining them directly with the Transition metal elementary metal powder :

eg. T.M.0 + MO3 ----- (heat , inert atm.) -----> (T.M.)2+M4+O3 ,

where T.M. = Mn , Fe , Co , Ni , and Cu ; M = Mo and W .

Cr(IV) is a very powerful oxidizer (E0red = 1.48 V to Cr3+) and would oxidize the (T.M.)2+ to (T.M.)3+, except for Co2+ and Cu2+ [CoCrO3 and CuCrO3 are discussed in a separate section below] . Higher-valent molybdenum and tungsten species are much weaker oxidizers by comparison and wouldn't affect the (T.M.)2+ cations in the ilmenites .

Copper(0) [ 0.3419 V to Cu2+] should be satisfactory for Mo(VI) [0.700 V to Mo(V)] , but probably not for tungsten(VI) [0.26 V to W(V)] .

In the series of doped composites , LiMoO3 and LiWO3 , generated in situ , could be the dopants with emptyMo(VI) and W(VI) respectively :

eg. x Co0 + (1-x) Li2CO3 + 1/6(1-x) Mo0 + 1/6(x+5) MoO3 ----- (heat , flowing inert atm.) ----->

CoxLi1-xMoO3 + (1-x) CO2 (g) ; x = 0 to 1 .

Both series of compounds , (T.M.)2+M4+O3 and (T.M.)xLi1-xMO3 , should be metallic solids with the ilmenite crystal structure . Again , we hope that the T.M. oxide layers will be AFM , and will be able to induce some AFM ordering into their metallic bond free electrons , thus promoting the appearance of superconductivity in them at elevated temperatures .


Manganese and Rhenium Ilmenites


NiMnO3 has the ilmenite crystal structure and is said to be ferrimagnetic . Is its internal charge distribution Ni2+Mn4+O3 , Ni3+Mn3+O3 , or Ni4+Mn2+O3 ? Consider the following redox equations :

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

MnO2 + 4 H+ + 2e- -------------> Mn2+ + 2 H2O ; E0red = 1.224 V ;

Mn3+ + e- -------------> Mn2+ ; E0red = 1.5415 V ;

NiO2 + 4 H+ + 2e- -------------> Ni2+ + 2 H2O ; E0red = 1.678 V .

The order of the oxidizing strengths of the four possible Transition metal species involved is therefore : Ni(IV) > Mn3+ > Mn(IV) > Ni3+ , leading to the most likely compromise charge distribution of Ni3+Mn3+O3 . Since E0red is greater for Mn3+ than Ni3+ , the former would probably have more covalent character than the latter . It would thus be appropriate to call NiMnO3nickel(III) manganate.

The trivalent nickel in , and the ferrimagnetic nature of NiMnO3 make it valueless as a superconductor candidate material . A2+Mn4+O3 , with an A2+O AFM induction layer , would be more desireable . The Co2+ in CoO (TN = 291 K) , with a E0red = 1.92 V , should be satisfactory ; it won't be oxidized by the Mn(IV) in Co2+Mn4+O3 . Manganese dioxide is a semiconductor-like pseudometal , with an ambient electrical conductivity of ~ 20 ohm-1cm-1 (Bowen's metal oxides conductivity graph , above) . Vast amounts of black industrial MnO2 are used as the oxidizer in throwaway dry cell electrical batteries .

Manganese(IV) is 3d3 electronically , and the octahedrally coordinated Mn will probably use a d5s hybrid atomic orbital for its six MnO covalent bonds . The 3d valence electrons will therefore be promoted into the 4px,y,z frontier orbitals , which can form a bilayer MnO metallic bond in the lattice . We can reasonably expect CoMnO3 to be a metallic solid , and if the AFM ordering in the crystal is of the intralayer variety , it could even be superconducting at a respectable temperature .

CoMnO3 might be prepared by a solid state synthesis :

CoCO3 + MnO2 ------- pure flowing oxygen atmosphere , heat ---------> CoMnO3 + CO2 (g) .

The furnace atmosphere of pure , flowing oxygen is required to sweep out the by-product carbon dioxide gas , and to retard the decomposition of the manganese dioxide , which expels oxygen at 535 C and is autoreduced to manganese(III) oxide .

Mixed-valent Mn(IV)Mn(V) composites might also be synthesized and studied . Part of the Mn(IV) would be oxidized to Mn(V) by lithium or sodium peroxide :

x CoCO3 + (1-x) Li2O2 + MnO2 ------- pure flowing oxygen atmosphere , heat ---------> CoxLi1-xMnO3 + x CO2 (g) , where x = a mole ratio taken experimentally between 0 and 1 by the researcher .

Note : I see in my Alfa-Aesar catalog that lithium and sodium peroxides are available only at 90% and 93% purities , respectively . An accurate chemical analysis of the reagent on hand for actual lithium or sodium content might be advisable . The exact quantity of solid peroxide to be used in the preparation can then be calculated , based on Li or Na , not the peroxide , since the quantity of LiMnO3 (or NaMnO3) created in the reaction pellet is critical . The slightly deficient oxygen content in the mixture could be augmented by the pure oxygen in the furnace atmosphere .

Doped copper(II) manganates , CuxLi1-xMnO3 , could also be studied . Mn(IV) is incapable of oxidizing Cu(II) to Cu(III) [at – 2.4 V] , so Cu2+Mn4+O3 should be redox-stable , as well as its lithium-doped composites : x CuO + (1-x) Li2O2 + MnO2 ------- (etc.) ---------> CuxLi1-xMnO3 .

Similar considerations apply to rhenium analogues of CoxLi1-xMnO3 . However , Re(IV) is a very mild oxidizer ,

ReO2 + 4 H+ + 4e- -------------> Re0 + 2 H2O ; E0red = 0.2513 V .

It should therefore be possible to synthesize the M2+Re4+O3 series of rhenium ilmenites , with M = Mn , Fe , Co , Ni , and Cu , and the Re(IV)–Re(V) mixed-valent series , MxLi1-xReO3 .

The key component in these materials would be ReO2 , a gray crystalline material , reasonably stable (dec. ~ 900 C) , and a metallic solid (s ~ 50 ohm-1cm-1 , ambient , Bowen's Figure 5 , above) . Its derivatives should similarly be metallic and possibly superconducting if the AFM ordering in them is of the intralayer variety . Rhenium dioxide is commercially available , but like rhenium metal itself and all rhenium compounds , it is very expensive (rhenium is the costliest of all the base metals) .


CuCrO3 and CoCrO3


Copper(II) oxide derivatives have enjoyed great success as AFM induction agents in all of the known HTS materials (eg. BSCCO-2212 , sketch above) . It might be possible to use Cu2+ in an ilmenite-like heterostructure if permitted by redox considerations . Note , however , that Cu2+ is affected by Jahn-Teller distortion , so that the resulting Cu2+M4+O3 compound would have elongated CuO bonds and a pronounced layered structure . To the best of my knowledge copper(II) never has a symmetrical octahedral coordination by any ligands . When bonded with oxygens , it always has a square planar , square pyramidal , or tetragonal (4+2 distorted octahedral) coordination by them . The latter six-fold coordination would likely occur around the Cu2+ by the oxygens in the hypothetical Cu2+M4+O3 compounds .

As mentioned above , chromium dioxide is a metallic solid with an excellent electrical conductivity over a wide temperature range (~ 2500 ohm-1cm-1 at room temperature , rising to ~ 50,000 ohm-1cm-1at 100 K) . Its ferromagnetic electrons undoubtedly prevent it from becoming a superconductor at lower temperatures . Could we combine CrO2 with CuO to form an ilmenite-like heterostructure : CuO + CrO2 ------> CuCrO3 , in which the metallic CrO layers alternate with the powerfully AFM CuO layers ?

A copper chromite compound , which apparently has the approximate formula CuCr2O4 , is well-known in organic chemistry , where it has been used as a hydrogenation (mostly) and dehydrogenation (occasionally) catalyst . When its preparation includes an equimolar quantity of the barium analogue , BaCr2O4 , it is known as Lazier's catalyst after its discoverer .

As the somewhat obscure chromium dioxide is difficult to access , perhaps a more practical route to CuCrO3 could involve the much better known and readily available chromium trioxide . As with the molybdenum and tungsten ilmenites discussed above , CuCrO3 might be synthesized by the combination of elementary Cu0 and CrO3 . Let's look at the redox chemistry involved :

Cu0 – 2e- -------------> Cu2+ ; E0ox = – 0.3419 V ;

Cr(VI) + 2e- -------------> Cr(IV) ; E0red = 1.34 V ; [Note]

Net reaction : Cu0 + Cr(VI) -------------> Cu2+ + Cr(IV) ; E0T = ~ 1.00 V .

Note : This Cr reduction reaction is actually Cr(V) + e- ------> Cr(IV) , as I couldn't find the E0red value for the Cr(VI) / Cr(IV) reduction . However , as is well known , CrO3 is a powerful oxidizing agent , so the E0red = 1.34 V value shown for it is probably about right .

The overall reaction potential of E0T = ~ 1.00 V indicates that the Cu0 + CrO3 ------> CuCrO3 reaction would likely be both thermodynamically spontaneous at STP , and quite exothermic . Because it's an oxidizing system , the reactants and products wouldn't require any elaborate sort of protection from atmospheric oxidation , unlike the other synthetic ilmenites discussed above .

Any researcher attempting this reaction would be advised to : (1) use only semi-micro quantities of reagents , only a few grams , perhaps at the 0.01 mole scale ; (2) observe all available safety measures : eye protection by shatterproof safety glasses is imperative ; (3) a shatterproof safety visor for the face and/or a safety shield would be advisable ; (4) heavy leather gloves or asbestos mittens should be worn to protect the hands . The reaction mixture of copper powder and finely-ground chromium trioxide would be placed in a suitably-sized porcelain crucible , which is held in a clay-pipe triangle on a stand . The researcher would cautiously heat the crucible with a Bunsen or Meker burner , or a propane torch , until some indication of the commencement of the reaction was observed . It might also be a good idea to conduct the reaction outside , on the lawn in back of the laboratory , and have a fire extinguisher on hand . Be very careful !

The main concern with CuCrO3 is that Cr(IV) is a metastable chromium valence ; the three stable chromium valences (apart from zerovalent Cr metal itself , of course) are the oxidizing Cr(VI) , the reducing Cr2+, and the low energy Cr3+. Thus , unless CuCrO3 is energetically stabilized in a “comfortable” crystal structure hopefully ilmenite-like it might disproportionate during synthesis into more thermodynamically stable forms , such as Cr2O3 and Cu(CrO4)2 . The Wikipedia Chromium trioxide web page notes :

“The trioxide reacts with cadmium , zinc , and other metals to generate passivating chromate films that resist corrosion”. The CrO3 might actually be reacting with the extremely thin oxide coating on these metals , and not necessarily with the zerovalent atoms underneath .

CrO3 itself isn't very stable ; it melts at 197 C and decomposes at ~ 250 C (to Cr2O3 and O2) . A hot , violent reaction might promote this decomposition and a disproportionation reaction . Consideration should therefore be given to designing a chemie douce synthesis of CuCrO3 .

CrO3 forms complexes with certain organic reagents , eg. pyridine , which dissolve in organic solvents . Preparation of a stable , synthetically useful solution of the CrO3–pyridine complex in methylene chloride has been described by Ratcliffe and Rodehorst (1970) . This deep burgundy solution was considered the reagent of choice in almost all situations calling for the oxidation of an alcohol (p. 4001) .

The second reagent solution would contain a copper(I) reactant , also in the form of a complex with organic ligands , dissolved in an organic solvent . Copper(I) chloride is known to form many complexes with nitrogen atom lone pairs in amines (such as pyridine) and nitriles (such as acetonitrile) . It would be most convenient if the (Py)2CuCl complex was soluble in CH2Cl2 ; however , if this wasn't the case perhaps a (Et3N)2CuCl or (n-Bu3N)2CuCl complex might be satisfactory .

The CrO3–pyridine complex (1.0 eq.) in CH2Cl2 would be added dropwise , with mechanical stirring and with cooling if necessary , to the copper(I) complex solution (2.0 eq.) in CH2Cl2 :

2 CuCl complex + CrO3–pyridine -------------> CuCrO3 (ppt) + CuCl2–pyridine .

The Cr(VI) atom would oxidize both Cu(I) atoms to Cu(II) [E0ox = 0.153 V] . One Cu2+ would bond with the reduced [CrO3]2- to form the insoluble precipitate CuCrO3 . The second Cu(II) would scavenge the chloride anions to form the complexed by-product CuCl2–pyridine . Hopefully the reduction wouldn't skip lightly over Cr(IV) and end up at the low energy Cr3+. The CuCrO3 precipitate would be washed with several aliquots of CH2Cl2 , then dried at room temperature and maybe in a drying oven at ~ 110 C ; however , heating or calcination at any temperature higher than this would be inadvisable .

Copper(III) is the most powerfully oxidizing metal cation known (Cu3+ + e- ------> Cu2+ ; E0red = 2.4 V) ; conversely , Cu2+ is the most difficult metal cation to oxidize to its higher oxidation state . Chromium(IV) is also strongly oxidizing (CrO2 + 4 H+ + e- ------> Cr3+ + 2 H2O ; E0red = 1.48 V) , but it isn't strong enough to oxidize Cu2+ . Nor is the Cu2+ able to oxidize the Cr(IV) , so the electronic condition inside CuCrO3 would remain stable as Cu2+Cr4+O3 . However , CuCrO3 should be considered as being metastable and susceptible to decomposition to disproportionated Cr3+ and Cr(VI) products , such as Cr2O3 and Cu(CrO4)2 , if it's heated to any substantial temperature .

Chromium(IV) is 3d2 electronically , but as in CrO2 these two electrons would be promoted into the chromium 4p orbitals (GIF image , 48 KB) when their 3d orbitals are “taken” by the d5s octahedral hybrid orbital for the six CrO covalent (not ionic) bonds . Without knowing the coordination of the oxygens in CuCrO3 , I can't predict its full electronic structure other than to suggest that it would probably have a 4p pi XO metallic bond .

The synthesis and study of other copper ilmenites would be equally interesting , but with uncertainty about their internal redox conditions it would be difficult to predict what sort of products would be obtained :

eg. Cu0 + MoO3 ----- (heat , inert atmosphere) -----> Cu2+Mo4+O3 ;

but possibly , Cu2+Mo4+O3 ----------> Cu1+Mo5+O3 .

The Mo5+O3 layers might be metallic as in the bronzes , but the Cu1+ (3d10) would be diamagnetic and thus not AFM at all . The copper(I) cations in Cu1+Mo5+O3 would have a linear coordination by the oxides , as opposed to the tetragonal coordination of copper(II) by oxides in Cu2+Mo4+O3 .

If CuCrO3 can be synthesized with an ilmenite-like structure , and is electronically stable [that is , its Cr(IV) doesn't disproportionate to Cr(III) and Cr(VI)] , it could prove to be an exceptional superconductor candidate compound and well worth the effort to prepare and study .

On the other hand , it might be metallic but never superconducting . As an ilmenite it would have hexagonal packing , which is prone to support the interlayer type of AFM magnetic structure . The compound YCuO3 is also known to be metallic ; it has a hexagonal structure and is probably an ilmenite . Arjomand and Machin (2) measured its magnetic susceptibility as 3.01 BM at 300 K , and 2.10 BM at 80 K (liquid nitrogen) . YCuO3 seems to be antiferromagnetic , but it obviously isn't superconducting in liquid nitrogen . I suspect it's very much like the highly metallic , hexagonal FeSn and FeP , which never become superconducting . Like them , YCuO3 very likely has an interlayer type of AFM ordering . The hypothetical ilmenite CuCrO3 could behave in a similar manner to these other hexagonal compounds .

The related compound CoCrO3 [Co2+Cr4+O3] should be redox-stable , since Cr(IV) [1.48 V to Cr(III)] would be incapable of oxidizing Co2+ to Co3+ (1.92 V) . It might be synthesized by the highly energetic reaction , Co0 + CrO3 ---------> CoCrO3 . This reaction could be moderated by adding the finely-divided cobalt metal powder slowly , and with vigorous mechanical stirring and cooling , to a solution of the CrO3pyridine complex in methylene chloride . Hopefully this carefully controlled technique might also minimize a side-reduction of the Cr(IV) to the undesireable Cr(III) . As with CuCrO3 , CoCrO3 must also be considered a metastable material and be produced and processed at moderate temperatures .

It might also be possible to synthesize CoCrO3 by a chemie douce method , for example by the reaction of the CrO3pyridineCH2Cl2 solution with anhydrous CoCl2 and a sacrificial reducing agent such as TDAE [tetrakis(dimethylamino)ethylene] . Anhydrous CoCl2 , described as blue hygroscopic leaflets, m.p. 737 C , is soluble in a variety of organic solvents (EtOH , ethyl ether , THF , acetone , and pyridine) . TDAE is a remarkably strong one- or two-electron oxidizer ; it's commercially available (eg. Aldrich Chemical Co.) . The CrO3 would act as a two-electron oxidizer , oxidizing the TDAE0 to TDAE2+ , which would scavenge the chloride anions from the CoCl2 :

CoCl2 / polar organic solvent + CrO3pyridineCH2Cl2 ------- (add TDAE dropwise at 25 C or with cooling , mechanical stirring ) --------> Co2+ [CrO3]2- (ppt) + TDAE2+(Cl-)2 .

A similar technique could be tried for CuCrO3 , using anhydrous copper(II) chloride , which is quite soluble in polar organic solvents , and forms a stable complex with pyridine :

CuCl2 / polar organic solvent + CrO3pyridineCH2Cl2 ------- (add TDAE dropwise at 25 C or with cooling , mechanical stirring ) --------> Cu2+ [CrO3]2- (ppt) + TDAE2+(Cl-)2 .


Copper and Cobalt Iron(IV) and Lead(IV) Ilmenites


Barium ferrate , BaFeO3 , is a ferromagnetic perovskite with hexagonal symmetry (bulk form ; metastable cubic form when prepared epitaxially) . Its iron(IV) atoms are 3d4 electronically . A related cubic perovskite , SrFeO3 , has an appreciable ambient electrical conductivity (~ 750 ohm-1cm-1 per H.K. Bowen , Figure 11 , p. 307 , electrical conductivity ; ~ 1000 ohm-1cm-1 per MacChesney , Sherwood , and Potter , for stoichiometric material) .

Iron(IV) is undoubtedly a powerful oxidizer , although I don't know what its E0red (to Fe3+) is . It might form an ilmenite-like compound with copper(II) , eg. Cu2+Fe4+O3 , which would be redox stable ; Cu2+ (at 2.4 V to Cu3+) is unaffected by any other metal cation oxidizer :

CuO + Fe2O3 + O2 (g) ----- (heat , pure oxygen atmosphere) -----> CuFeO3 , and

x CuO + (1-x) Li2O2 + Fe2O3 + O2 (g) ----- (heat , pure oxygen atmosphere) -----> CuxLi1-xFeO3 .

Lanthanum-doped CuFeO3 was studied by Souza and co-workers (2011) as a Fischer-Tropsch syngas catalyst .

The corresponding cobalt ferrates , Co2+Fe4+O3 and CoxLi1-xFeO3 , might also be redox stable and worthy of investigation .

Lead(IV) compounds are strongly oxidizing [Pb(IV) + 2 e- -----> Pb(II) , E0red = 1.455 V] , so only the cobalt(II) and copper(II) lead(IV) ilmenites would be redox-stable . Shannon and Bierstedt described the high temperature and hydrothermal syntheses of the metallic perovskite BaPbO3 (s = 3448 ohm-1cm-1 at 296 K) . The two ilmenite compounds CoPbO3 and CuPbO3 might similarly be prepared and studied :

CoCO3 + PbO + O2 (g) ----- (heat , pure flowing oxygen atmosphere) -----> CoPbO3 + CO2 (g) ;

CuO + PbO + O2 (g) ----- (heat , pure oxygen atmosphere) -----> CuPbO3 .


All of the ilmenite heterostructures proposed in this survey should be metallic solids with an appreciable electrical conductivity . Their included AFM layers have the potential of making them superconducting at higher temperatures . However , their hexagonal atomic packings may result in them having the wrong type of AFM ordering (interlayer) rather than the beneficial type (intralayer) . Interlayer AFM ordering , with its intralayer ferromagnetic electron spin arrangement , would effectively prohibit the appearance of superconductivity in the materials , even very close to Absolute Zero . Unfortunately , we can't predict in advance as to what sort of AFM order will be produced in the new ilmenite compounds . At this stage of the investigation the research must be trial-and-error, and relying on serendipity for promising results . I hope these considerations won't discourage solid state chemists from investigating any of the ilmenite systems surveyed above .

The redox chemistry discussed in this essay may look complicated and confusing at first glance ; however , it's really nothing more than simple High School chemistry redox equations , combined with chemical common sense. Even after a casual perusal of this study , though , I think the reader will agree with me that the research and development of rationally-designed HTS materials most assuredly lies well within the realm of solid state chemistry , and no longer in condensed matter physics . The chemistry to actually produce the new materials MUST come first . Then the physics , to study and test the physical properties of the materials , will follow . The responsiblity and expectations for the future development of HTS heterostructures are thus with chemists , whose creativity and ingenuity will be exercised to the fullest extent in their exploration .


References and Notes


All the standard redox potentials cited are versus the SHE at STP , and were from : D.R. Lide (ed.) , CRC Handbook of Chemistry and Physics , 87th edition , CRC Press / Taylor & Francis , Boca Raton (FL) , 2006 ; P. Vansek (ed.) , “Electrochemical Series”, pp. 8-20 to 8-25 . They were derived either from measurements in aqueous media or from thermodynamic calculations , so we should be cautious about applying them in a solid state environment . Nevertheless , they can provide some useful guidance to the chemist in the design of new superconductor candidates . For a convenient tabulation of oxidizing metal oxides and their E0red values , see this GIF image (45 KB) .

Also from the CRC Handbook of Chemistry and Physics : H.P.R. Frederikse (ed.) , “Selected Antiferromagnetic Solids”, pp. 12-107 to 12-108 .

Chemexplore web pages : “Antiferromagnetic Induction in High Temperature Superconductors” ; “New Layered Compounds for High Temperature Superconductivity” ; “A Rhenium Trioxide–Copper Oxide Layer Compound” ; “A Survey of Superconductors”.

Fermi-Dirac distribution : A.R. Mackintosh , “The Fermi Surface of Metals”, Scientific American 209 (1) , pp. 110-120 (July , 1963) ; also in Moore's textbook [below] , Ch. 2 , “Gold”, pp. 41-72 ; see Fig. 2.4 , p. 49 for a sketch of a typical Fermi-Dirac distribution curve .

Nolting and co-workers : 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] ; “PEEM2 Reveals Spin Alignment in Magnetic Layers” (web page) ; F. Nolting et al. , “Observation of Antiferromagnetic Domains in Epitaxial Thin Films”, [PDF , 568 KB] ; 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] ; 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] ; 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] .

Awana and co-workers : R. Jha , K. Singh , and V.P.S. Awana , “Synthesis and Superconductivity of New BiS2 Based Superconductor PrO0.5F0.5BiS2”, (August 29 , 2012) [PDF , 154 KB] ; R. Jha , A. Kumar , S.K. Singh , and V.P.S. Awana , “Superconductivity at 5K in NdO0.5F0.5BiS2”, (August 18 , 2012) [PDF , 972 KB] ; V.P.S. Awana et al. , “Appearance of Superconductivity in New BiS2 Based Layered LaO0.5F0.5BiS2”, (August 1 , 2012) [PDF , 736 KB] ; S.K. Singh et al. , “Bulk Superconductivity in Bismuth-Oxy-Sulfide Bi4O4S3”, (October 10 , 2012) [PDF , 647 KB] .

Cava and co-workers : R.J. Cava et al. , “Superconductivity Near 30 K Without Copper : the B0.6K0.4BiO3 Perovskite”, Nature 332 (6167) , pp. 814-816 (1988) ; L.F. Mattheis , E.M. Gyorgy , and D.W. Johnson , Jr. , “Superconductivity Above 20 K in the Ba–K–Bi–O System” , Phys. Rev. B 37 (7) , pp. 3745-3746 (1988) .

Maeda and co-workers : H. Maeda et al. , “A New High Tc Oxide Superconductor Without a Rare Earth Element”, Jpn. J. Appl. Phys. (Lett.) 27 (2) , pp. L209-L210 (1988) ; J.M. Tarascon et al. , “Preparation , Structure , Properties of the Superconducting Compound Series Bi2Sr2Can-1CunOy with n = 1, 2, and 3”, Phys. Rev. B 38 (13) , pp. 8885-8892 (1988) .

undoubtedly the classic BCS : B. Li and Z.W. Xing , “Electronic Structure and Superconductivity of BiS2-Based Compounds LaO1-xFxBiS2”, (October 5 , 2012) [PDF , 2058 KB] .

family of compounds : R.W.G. Wyckoff , Crystal Structures , 2nd edition , vol. 2 , Interscience (John Wiley) , New York (1964) ; pp. 420-422 ; A.F. Wells , Structural Inorganic Chemistry , 3rd edition , Clarendon Press , Oxford (UK) , 1962 ; p. 486 ; X.C. Liu , R. Hong , and C. Tian , “Tolerance Factor and the Stability Discussion of ABO3-Type Ilmenite”, J. Mater. Sci. : Mater. Electron. 20 (4) , pp. 323-327 (2009) .

neutron diffraction : C.G. Schull , W.A. Strauser , and E.O. Wollan , “Neutron Diffraction by Paramagnetic and Antiferromagnetic Substances”, Phys. Rev. 83 (2) , pp. 333-345 (1951) [the magnetic structure of rocksalt MnO , identical to that of NiO , is shown in their Fig. 5 , p. 338] ; Y.-Y. Li , “Magnetic Moment Arrangements and Magnetocrystalline Deformations in Antiferromagnetic Compounds”, Phys. Rev. 100 (2) , pp. 627-631 (1955) ; W.L. Roth , “Multispin Axis Structures for Antiferromagnets”, Phys. Rev. 111 (3) , pp. 772-781 (1958) ; H.A. Alperin , “Neutron Diffraction Investigation of the Magnetic Structure of Nickel Oxide”, J. App. Phys. 31 (5 suppl.) , pp. 354S-355S (1960) .

ferromagnetic : K. Yamaguchi and H. Watanabe , Neutron Diffraction Study of FeSn, J. Phys. Soc. Jpn. 22 (5) , pp. 1210-1213 (1967) ; L. Hggstrm , T. Ericsson , R. Wppling , and K. Chandra , “Studies of the Magnetic Structure of FeSn Using the Mossbauer Effect”, Physica Scripta 11 (1) , pp. 47-54 (1975) .

excellent electrical conductor : B. Stenstrm , “The Electrical Resistivity of FeSn Single Crystals”, Physica Scripta 6 (4) , pp. 214-216 (1972) .

iron monophosphide : D. Bellavance , M. Vlasse , B. Morris , and A. Wold , “Preparation and Properties of Iron Monophosphide”, J. Solid State Chem. 1 (1) , pp. 82-87 (1969) ; see Figures 3 and 4 , p. 86 , for temperature–conductivity graphs . See also D. Bellavance and A. Wold , “Single Crystals of Iron Monophosphide”, Inorg. Synth. 14 , pp. 176-182 , A. Wold and J.K. Ruff (eds.) , McGraw-Hill , New York , 1973 .

Moore's textbook : 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 .

magnetic structures of the four titanate ilmenites : G.S. Heller , J.J. Stickler , S. Kern , A. Wold , “Antiferromagnetism in NiTiO3”, J. App. Phys. 34 (4 , Pt. 2) , pp. 1033-1034 (1963) ; J.J. Stickler , S. Kern , A. Wold , and G.S. Heller , “Magnetic Resonance and Susceptibility of Several Ilmenite Powders”, Phys. Rev. 164 (2) , pp. 765-767 (1967) ; G. Shirane , S.J. Pickart , and Y . Ishikawa , “Neutron Diffraction Study of Antiferromagnetic MnTiO3 and NiTiO3”, J. Phys. Soc. Jpn. 14 (10) , pp. 1352-1360 (1959) [these authors sketched three possible magnetic structures for the AFM titanate ilmenites . In their Fig. 1 , p. 1353 , model (a) is the interlayer type (FeTiO3 and NiTiO3 ) ; model (c) is the intralayer type (MnTiO3 ) ; and model (b) represents an intralayer ordering but with a different stacking of the layers than model (c)] ; R.E. Newnham , J.H. Fang , and R.P. Santoro , “Crystal Structure and Magnetic Properties of CoTiO3”, Acta Cryst. 17 , Part 3 , pp. 240-242 (1964) ; anon. , “Magnetic and Structural Properties of Hemo-Ilmenite Solid Solutions” [web page] .

Arjomand and Machin (1) : M. Arjomand and D. J. Machin , “Oxide Chemistry . Part I . Ternary Oxides Containing Nickel in Oxidation States II , III , and IV”, J. Chem. Soc. Dalton Trans. 1975 (11) , pp. 1055-1061 .

electrical conductivity : H.K. Bowen , “Ceramics as Electrical Materials”, pp. 290-314 in the Kirk-Othmer Encyclopedia of Chemical Technology , 3rd edition , Vol. 5 , M. Grayson and D. Eckroth (eds.) , John Wiley , New York (1979) ; Figure 5 , p. 299 .

Johnston : D.C. Johnston , “Superconducting and Normal State Properties of Li1+xTi2-xO4 Spinel Compounds . I . Preparation , Crystallography , Superconducting Properties , Electrical Resistivity , Dielectric Behaviour , and Magnetic Susceptibility”, J. Low Temp. Physics 25 (1&2) , pp. 145-175 (1976) .

Donohue : P.C. Donohue , “The Synthesis , Structure , and Superconducting Properties of New High-Pressure Forms of Tin Phosphide”, Inorg. Chem. 9 (2) , pp. 335-337 (1970) .

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] .

mercury cuprate : P. Dai et al. , “Synthesis and Neutron Powder Diffraction Study of the Superconductor HgBa2Ca2Cu3O8 by Tl Substitution”, Physica C 243 (3&4) , pp. 201-206 (1995) ; M. Cantoni , A. Schilling , H.-U. Nissen , and H.R. Ott , “Characterization of Superconducting Hg–Ba–Ca–Cu–Oxides , Structural and Physical Aspects”, Physica C 215 (1&2) , pp. 11-18 (1993) .

said to be ferrimagnetic : E.F. Bertaut and F. Forrat , “Structure and Ferrimagnetism of the Ilmenite Compound MnNiO3”, J. App. Phys. 29 (3) , pp. 247-248 (1958) .

Jahn-Teller distortion : P.A. Cox , Transition Metal Oxides , An Introduction to Their Electronic Structure and Properties , Clarendon Press , Oxford (UK) , 1995 :

“It [Jahn-Teller distortion] is nearly always found with two specific electron configurations , d4 (high-spin only) and d9 ” (p. 19) .

The chemistry explanation for Jahn-Teller distortion is quite simple : a singlet valence electron is located in the spatially voluminous axial dz2 orbital . This negatively-charged region of interatomic space is repellent toward approaching negatively-charged ligands such as anions and lone pairs of electrons . Transition metal cations with a singlet valence electron in a dz2 orbital (eg. high spin Cr2+, 3d4 ; and low spin [square planar] Cu2+, 3d9 ) are , in effect , oval-shaped , not spherical , as are most metal cations .

chromium dioxide : B.L. Chamberland , “The Chemical and Physical Properties of CrO2 and Tetravalent Chromium Oxide Derivatives”, CRC Crit. Rev. Solid State Mater. Sci. 7 (1) , pp. 1-31 (1977) . Chromium dioxide is also discussed by J.B. Goodenough , “Transition Metal Oxides with Metallic Conductivity”, Bull. Soc. Chim. France (4) , pp. 1200-1206 (1965) ; pp. 1204-1205 .

Lazier's catalyst : W.A. Lazier and H.R. Arnold , “Copper Chromite Catalyst”, Org. Syn. Coll. Vol. 2 , pp. 142-145 (1943) [PDF , 128 KB] ; L. F. Fieser and M. Fieser , Reagents for Organic Syntheses , vol. 1 , John Wiley , New York , 1967 ; pp. 156-157 .

chemie douce synthesis : The high temperature solid state (shake-'n-bake) synthesis of MnTiO3 , FeTiO3 , CoTiO3 , and NiTiO3 from simple precursor chemicals was discussed by Stickler and co-workers (magnetic structures of the four titanate ilmenites , above) . A chemie douce preparation of NiTiO3 was described by P.-H. Yuan et al. , “Preparation and Photocatalytic Properties of Ilmenite NiTiO3 Powders for Degradation of Humic Acid in Water”, Int. J. Miner. Metall. Mater. 19 (4) , pp. 372-376 (2012) . In a remarkably simple room temperature procedure , ethanol solutions of tetrabutyl titanate (with added citric acid) and nickel(II) acetate were mixed together . NiTiO3 particles immediately precipitated , and the heterogenous mixture was warmed at 70 C for hr to complete the separation of the light green product . This material was washed , dried , and calcined to provide pure NiTiO3 . A somewhat more complicated chemie douce synthesis of NiTiO3 was described by G.A. Traistaru et al. , “Synthesis and Characterization of NiTiO3 and NiFe2O4 as Catalysts for Toluene Oxidation”, Dig. J. Nanomater. Biostruct. 6 (3) , pp. 1257-1263 (2011) [PDF , 255 KB] .

Ratcliffe and Rodehorst : R. Ratcliffe and R. Rodehorst , “Improved Procedure for Oxidations with the Chromium Trioxide-Pyridine Complex”, J. Org. Chem. 35 (11) , pp. 4000-4002 (1970) ; R.W. Ratcliffe , “1-Decanal”, Org. Syn. Coll. Vol. 6 , pp. 373-375 (1988) [PDF , 142 KB] .

complexes with nitrogen atom lone pairs : S. Ahrland , “Complex Equilibria , Solvation and Solubility”, Pure & Appl. Chem. 62 (11) , pp. 2077-2082 (1990) [PDF , 407 KB] ; CuX solvation with DMSO , acetonitrile , and pyridine ligands is discussed ; K. Nilsson and A. Oskarsson , “Structural Relationships Among Solvates of Copper Halides . The Structure of Catena-di-m-iodo-acetonitrile-copper(I) at 200 K”, Acta Chem. Scand. A39 (9) , pp. 663-666 (1985) [PDF , 319 KB] ; A.U. Malik , “Coordination Compounds of Copper(I) Chloride with Pyridine and Related Ligands”, Z. Anorg. Allg. Chem. 344 (1-2) , pp. 107-112 (1966) :

“Complexes of Cu(I) with pyridine , picolines , lutidines , quinolines , piperidine , and acridine have been studied by the potentiometric method . Only the complexes of 3-picoline , 2,4- and 2,6-lutidines , quinoline , piperidine , and acridine could be isolated in pure form” [abstract] ;

J.L. Roubaty , A. Revillon , and M. Breant , “A Polarographic Study of the Pyridine–Copper–Chloride Complexes in Methanol and Determination of the Stability Constants”, Talanta 24 (11) , pp. 688-690 (1977) :

“Copper(I) chloride has been shown to form two complexes with pyridine in anhydrous methanol , CuClPy and CuClPy2 ” [abstract] .

Copper(I) chloride also forms many coordinate covalent compounds with olefins ; for example :

This sketch was copied from the Wikipedia web page , Copper(I) Chloride. I thank the author of this illustration , and Wikipedia , for implied permission to reproduce it here .

This copper(I) complex , di-m-chloro-bis(cis,cis-1,5-cyclooctadiene)dicopper(I) , was originally prepared by H.L. Haight , J.R. Doyle , N.C. Baenziger , and G.F. Richards , “Metal-Olefin Compounds . III. Some Compounds of Copper(I) Containing Cyclic Olefins”, Inorg. Chem. 2 (6) , pp. 1301-1303 (1963) . These copper(I)olefin compounds might also be suitable as the Cu(I) reagent in the reaction with the CrO3–pyridine complex in a chemie douce synthesis of CuCrO3 . Another diolefin that might readily form a coordinate covalent compound with CuCl is the highly reactive cyclopentadiene (b.p. 42 C) .

Arjomand and Machin (2) : 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 ; magnetic susceptibility on Table 3 , p. 1062 ; discussion of YCuO3 on pp. 1065-1066 . These authors also studied BaCuO2 and found it to be weakly AFM , as mentioned above . The combination of the metallic YCuO3 layers with the non-metallic but AFM BaCuO2 layers produced the first genuine HTS heterostructure : YCuO3 + 2 BaCuO2 -------> (YCuO3)(BaCuO2)2 , ie. YBa2Cu3O7 , YBCO .

TDAE : R.L. Pruett et al. , “Reactions of Polyfluoro Olefins. II. Reactions with Primary and Secondary Amines”, J. Amer. Chem. Soc. 72 (8) , pp. 3647-3650 (1950) ; the preparation of TDAE [b.p. 59 C (0.9 mm) , strongly luminescent with air] is described on p. 3649 . TDAE is commercially available , eg. from Aldrich .

one- or two-electron oxidizer : R.W. Hoffmann , “Reactions of Electron-Rich Olefins”, Angew. Chem. Internat. Ed. Engl. 7 (10) , pp. 754-765 (1968) ; N. Wiberg , “Tetraaminoethylenes as Strong Electron Donors”, Angew. Chem. Internat. Ed. Engl. 7 (10) , pp. 766-779 (1968) . TDAE's reducing strength is said (Hoffmann , p. 756) to be comparable to that of zinc metal (Eox0 = 0.762 V) .

Barium ferrate : N. Hayashi et al. , “BaFeO3 : A Ferromagnetic Iron Oxide”, Angew. Chem. Internat. Ed. Engl. 50 (52) , pp. 12547-12550 (2011) ; C. Callender et al. , “Ferromagnetism in Pseudocubic BaFeO3 Epitaxial Films”, Appl. Phys. Lett. 92 (1) , 012514 , 3 pp. (2008) [PDF , 407 KB] .

MacChesney , Sherwood , and Potter : J.B. MacChesney , R.C. Sherwood , and J.F. Potter , “Electric and Magnetic Properties of the Strontium Ferrates”, J. Chem. Phys. 43 (6) , pp. 1907-1914 (1965) .

Souza and co-workers : J.R. Souza et al. , “LayCu0.4Fe0.6O3 Perovskite Oxides : Synthesis , Characterization , and Catalytic Reactivity in the Fischer-Tropsch Synthesis”, Brazil. J. Petro. Gas 5 (1) , pp. 1-9 (2011) [PDF , 914 KB] .

Shannon and Bierstedt : 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) .



[ Index Page ] [ Contact ]