High Temperature Superconductor Candidates Based on Modified La2CuO4 and La2NiO4
This web page is a continuation of the earlier Chemexplore web page , Ilmenites as High Temperature Superconductors [underlined blue hyperlinks can be clicked when online to retrieve the article cited . The requested document will open in a new window] . Interested readers will find the background to this essay in the Ilmenites study .
Various modifications of the strongly antiferromagnetic (AFM) compounds La2CuO4 and La2NiO4 as possible new high temperature superconductor (HTS) candidates will be surveyed in this presentation . The general method involves the design and synthesis of a heterostructure [multilayered sandwich compound ; also called a superlattice] having alternating covalent-metallic and non-metallic but strongly AFM layers . The AFM spin ordering is impressed on the mobile , free electrons above the Fermi level , EF , in the adjacent metallic bond , suppressing their Pauli paramagnetism and thereby assisting in the formation of Cooper pairs in the metallic layers as the material is cooled down .
The two AFM host structures of interest , La2CuO4 and La2NiO4 , were first synthesized by French researchers in 1960 (the references are presented at the end of the text , below) . La2NiO4 has the K2NiF4 crystal structure , while La2CuO4 has a distorted variant of it . In particular , La2CuO4 is tetragonally distorted ; its copper(II) atoms have a 4+2 distorted octahedral [tetragonal] coordination by the oxygen , with four short planar and two longer axial CuO bonds . This tetragonal distortion is caused by the Jahn-Teller effect , which is always observed in six-coordinate copper(II) oxide compounds :
In the following decades La2CuO4 and La2NiO4 were examined by many research groups and were found to be strongly antiferromagnetic , with Néel temperatures , TN , of ~ 315 K and 320-330 K , respectively . I should add that Foëx and co-workers (French researchers) also synthesized a related compound , La2CoO4 (TN = 275 K) , at the same time as La2CuO4 and La2NiO4 . Its crystal structure is similar , or related to , that of La2NiF4 . La2CoO4 could also conceivably be modified in the same way as I'll describe for La2CuO4 and La2NiO4 , but I consider it less useful than them . The analogous compounds La2MnO4 and La2FeO4 would probably also be AFM , but with lower Néel temperatures than La2NiO4 and La2CuO4 . TN values for the La2MO4 series would likely correlate fairly well with those of their corresponding binary oxides : MnO , 122 K ; FeO , 198 K ; CoO , 291 K ; NiO , 525 K ; and CuO , 230 K . CuO is anomalous because of its peculiar crystal structure .
La2CuO4 became especially interesting when the Swiss-German researchers Bednorz and Müller doped it with barium cations and discovered that the resulting composite containing mixed-valent copper(II,III) exhibited superconductivity at the remarkable transition temperature of Tc ~ 30 K . By substituting smaller strontium cations for the bariums , the compound (La0.925 Sr0.075)2CuO4 , with a somewhat higher Tc = 38 K , was obtained . These startling findings in 1986-87 launched the Cuprate Revolution in high temperature superconductivity .
In the following exposition La2CuO4 and La2NiO4 will also be modified to obtain possible HTS candidate compounds , but in a quite different manner than in Bednorz and Müller's doping procedure . In their doped La2CuO4 the metallic phase was derived from the host AFM phase , combined together in the same mass . However , La2CuO4 achieves its maximum AFM potential , as measured by its Néel temperature , 315 K , only when it's very pure and perfectly stoichiometric . Electron-doping or hole-doping La2CuO4 creates a mixed-valent state in the copper(II) cations , which causes their 3d valence electrons to rapidly resonate in the lattice . This resonance degrades the AFM spin ordering in the Cu(II) , and the Néel temperature of the doped material rapidly plummets from the 315 K maximum . The dilemma now becomes clear : hole-doping La2CuO4 with Ba2+ or Sr2+ is required to create the metallic Cu(II,III) mixed-valent state for electrical conduction ; but it simultaneously degrades the AFM ordering of the 3d valence electrons , lowers TN , and reduces the AFM induction effect in the resonating electrons .
Bednorz and Müller's doping method actually was a very inefficient way of combining AFM induction with a metallic solid . A much more efficient way to accomplish this objective is to have alternating , separate metallic and AFM induction layers in the compound . Electrical conductivity is optimized in the metallic layers , and AFM induction is optimized by ensuring that the AFM component is very pure and at its exact stoichiometric composition , to achieve its maximum Néel temperature . Then , its AFM induction effect will occur at the highest possible temperature , with the highest achievable Tc obtainable for that particular system. The rule of thumb here is : maximum electrical conductivity + maximum AFM induction = maximum superconductor Tc . That's what the compounds proposed in this report as possible HTS materials are intended to accomplish .
Tin(III) in LaSnNiO4
The strategy for the heterostructure synthesis is quite simple . La2NiO4 can be written as (La3+ La3+)(Ni2+)(O2-)4 to display the ionic valences involved . We can substitute a metallic M3+ for one of the La3+ cations ; or , the 2 + 4 combination of A2+ [electronically inert spectator cation] + M(IV) [covalent-metallic atom] can be substituted for both La3+ cations . The (Ni2+)(O2-)4 part of the formula remains the AFM induction layer in the composite .
A (1 + 5) combination of A1+ [electronically inert spectator cation] + M(V) [covalent-metallic atom] could also be tried within La2CuO4 and La2NiO4 host structures , but this report will focus only on heterostructures using MO2 covalent-metallic layers , ie. the (2 + 4) combination .
The hypothetical compound LaSnNiO4 illustrates the first method of designing a possible new HTS candidate material . A covalent-metallic layer of Sn3+ has been substituted for one layer of ionic La3+ in La2NiO4 . Actually , there's no such thing as Sn3+ ; however , the species Sn(III) does formally exist in the covalent-metallic compound tin(III) phosphide , SnP , first prepared by Donohue in 1970 . The tin atoms in SnP are really Sn(IV) + e- , with covalent SnP bonds and the extra electron , e- , delocalized in the metallic bond throughout the crystal lattice . SnP is an excellent electrical conductor at all temperatures . Its cubic rocksalt form is superconducting (Tc = 2.84.0 K) near Absolute Zero .
Thus , in LaSnNiO4 a covalent-metallic SnO layer is inserted into the strongly AFM La2NiO4 framework , with the ionic Ni2+O2- rocksalt layers inducing an AFM spin ordering pattern into the SnO metallic bond mobile , free electrons above EF . LaSnNiO4 can be thought of as consisting of SnO3 layers alternating with NiO layers , with electronically inert La3+ cations nesting in among the oxide anions and oxygen linking atoms , electrostatically bonded to them . Or , it can be considered as a combination of the hypothetical perovskite LaSnO3 with NiO into a multilayer heterostructure .
The heterostructures described in this web page , designed as HTS candidate materials , resemble in some aspects the Ruddlesden-Popper phases , which have the general formula An+1MnO3n+1 . They are comprised of n layers of a perovskite AMO3 with one layer of the corresponding AO rocksalt compound . Such Ruddlesden-Popper phases might be the perfect vehicle for new HTS heterostructures : the perovskite units (eg. LaSnO3) can provide the covalent-metallic layers , while the rocksalt units (eg. MnO , FeO , CoO , and NiO) can act as the nonmetallic , ionic AFM induction layers in the heterostructures .
An observation : n perovskites + 1 rocksalt = a Ruddlesden-Popper phase ; 1 ilmenite (or corundum) + 1 rocksalt = a spinel . Example : Fe2O3 (corundum) + FeO (rocksalt) = Fe3O4 (magnetite , an inverse spinel) . The compound AlSnNiO4 , with small metal atoms , might be a spinel and not a multilayer compound . Similarly for MgMNiO4 [next paragraph] . The larger La , Ba , and Sr cations in the proposed compounds will ensure that a perovskite and not an ilmenite will form the metallic layer in the proposed new heterostructures . They will then resemble Ruddlesden-Popper phases and not spinels .
The following sketch shows the approximate crystal structure of LaSnNiO4 ; I prepared it to illustrate the general family of BaM compounds , BaMNiO4 , based on modified La2NiO4 (the Ba can be replaced in most cases by Sr or even Ca , and in this compound by La) :
Note in the above sketch the direct linear link between the Sn atoms (small red spheres) and the nickel cations (small blue spheres) : SnONi . The AFM induction should be transmitted via electron superexchange through the orbitals probably the p orbitals of these particular atoms .
Magnetic induction of any sort in crystalline solids is accomplished orbitally , by an electron superexchange process . Cox comments ,
Direct through-space magnetic dipole interaction of spins is far too small to account for most magnetic-ordering phenomena (p. 148) .
Also note the possibility of having a bilayer SnO metallic bond in the compound . As discussed in several previous Chemexplore web pages , eg. the Ilmenites one , a bilayer metallic bond in an extended atomic lattice compound is essential for high temperature superconductivity in it , among other considerations . The sketch below presents a Valence Bond theory electronic structure for LaSnNiO4 , suggesting that it could well have a bilayer SnO metallic bond :
This VB analysis predicts that the extra , leftover electron from the lanthanum would be located in the tin atom's 6pz native atomic orbital , which could overlap continuously with the oxygen's 2pz2 orbital to form the tin 6pz1oxygen 2pz2 pi XO (XO = crystal orbital = polymerized molecular orbital = conduction band = metallic bond) in the lattice .
The extra , leftover electron from the lanthanum could also be located in the tin's 6s native atomic orbital , which could overlap continuously in the lattice with the oxygens' 2s2 orbitals to form a tin 6s1oxygen 2s2 sigma XO (not shown in the sketch due to space limitations) .
The combination of a tinoxygen bilayer metallic bond and strong AFM induction from the NiO layers could result in LaSnNiO4 having an exceptionally high superconductor transition temperature , possibly well above 100 K . Its synthesis should be fairly straightforward ; the required tin(III) might be produced from the reproportionation of tin(0) tin metal powder with the common and relatively inexpensive tin(IV) oxide , SnO2 :
½ La2O3 + ¼ Sn0 + ¾ SnO2 + NiO -------- [heat , nitrogen or argon atmosphere , or graphite layer] --------> LaSnNiO4 .
Tin(III) is a mild reducing agent , so the synthesis and processing of LaSnNiO4 should be carried out under an inert atmosphere of pure , dry nitrogen or argon . Alternately , the compressed pellet of reaction mixture could be placed in a graphite crucible and covered with a protective layer of unreactive graphite powder , which is gently tamped (or tapped) down . For additional protection a nitrogen or argon atmosphere could be used in the furnace :
Tin(III) is naturally metallic (at least in SnP) and requires no doping to become so . Nevertheless , LaSnNiO4 could be doped with the corresponding tin(IV) compound in varying mole ratios , to create electronically-active Sn(III,IV) mixed-valent composites . The electrically-insulating Sn(IV) analogue (Ba2+ Sn4+)(Ni2+)(O2-)4 could be synthesized and then used to dope LaSnNiO4 :
BaCO3 + SnO2 + NiO ------ (heat , flowing nitrogen atmosphere) -------> BaSnNiO4 + CO2 (g) ;
x LaSnNiO4 + (1-x) BaSnNiO4 -------- (mix , heat , etc. , N2 or Ar atm) -------> LaxBa1-xSnNiO4 ,
where x = a mole ratio taken experimentally between 0 and 1 by the researcher .
A dynamic (flowing) inert atmosphere over the reaction mixture flushes out the carbon dioxide effluent in the first reaction , helping to force the reaction to completion (le Chatelier's Principle) .
The analogous SrSnNiO4 could also be tried as a dopant . Arjomand and Machin(1) found that BaNiO2 and SrNiO2 were moderately antiferromagnetic , although they were unable to prepare the latter compound in pure form (ie. uncontaminated with NiO) . In solid state syntheses barium cations are frequently introduced into the reaction using the very pure reagent barium carbonate , BaCO3 . BaO , which is hygroscopic and reacts rapidly with atmospheric CO2 , is rarely used for these purposes . Similarly , SrO is obtained from SrCO3 . However , quite pure (99.5% , 100 mesh , m.p. 2430 ºC) SrO reagent is also commercially available , eg. from Alfa-Aesar . I've indicated its application in many of the following reactions .
On a practical note , most commercial nickel(II) oxide is of the black variety . This NiO is an inexpensive , industrial grade of the chemical , and appears black because it's a mixed-valent compound containing both Ni2+ and Ni3+ . The latter high-valent nickel is a fairly strong oxidizer (1.17 V to Ni2+) and obviously shouldn't be used with the reducing tin(III) and tin(III,IV) systems . The interested reader who would like to investigate LaSnNiO4 is advised to use only green NiO , which is the stoichiometric grade of the reagent with homovalent nickel(II) . Here's a picture of green nickel(II) oxide from the Internet :
(found on the web page , Huayou Cobalt ; my thanks to the copyright owner)
If a commercial supplier of pure , green reagent grade nickel(II) oxide can't be located , the researcher could prepare a sample quantity of it from : NiCl (aq) + 2 NaOH (aq) -------> Ni(OH)2 (ppt) + 2 NaCl (aq) ---- (filter ; wash ; dry ; heat under an inert nitrogen or argon atm) -----> NiO + H2O (g) .
The Néel temperature of the black , mixed-valent NiO is undoubtedly much lower than that of the stoichiometric , homovalent green variety , and that alone would make the former NiO undesireable as an effective AFM induction layer , even in chemically oxidizing systems .
The strongly AFM host structure La2CuO4 unfortunately can't be used together with Sn(III) , as Cu2+ is a mild natural oxidizer (Cu2+ + e- -------> Cu1+ ; E0ox = 0.153 V) . It would instantly oxidize Sn(III) to Sn(IV) [+ Cu(I)] . Both Sn(IV) insulator and Cu(I) diamagnetic would result in a dead BaM compound .
Titanium(III,IV) with La2NiO4
A titanium(III) compound can be doped with its titanium(IV) analogue to form a Robin-Day Class II mixed-valent compound , in which the Ti(III) 3d1 valence electron resonates extremely rapidly between the Ti(IV) kernels . 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) . Its Ti(III,IV) is evident in Li1+Ti3+ Ti4+ (O2-)4 , which is a Robin-Day Class II mixed-valent compound and a metallic solid .
As with the Sn(III,IV) composites mentioned above , the Ti(III,IV) mixed-valent BaM compounds might be prepared by combining varying mole ratios of a Ti(III) substrate , eg. LaTiNiO4 , with a Ti(IV) dopant , eg. SrTiNiO4 (the latter insulating dopant compound can be thought of as a combination of the cubic perovskite SrTiO3 and cubic rocksalt NiO , a quasi-Ruddlesden-Popper phase) :
½ La2O3 + ½ Ti2O3 + NiO -------- [heat , nitrogen or argon atmosphere , or graphite layer] --------> LaTiNiO4 (refer to the preparation of LaTiO3 by Kestigan and Ward) ;
SrO + TiO2 + NiO ------ [heat , nitrogen or argon atmosphere] -------> SrTiNiO4 ;
x LaTiNiO4 + (1-x) SrTiNiO4 -------- (mix , heat , etc. , N2 or Ar atm) -------> LaxSr1-xTiNiO4 ,
where x = a mole ratio taken experimentally between 0 and 1 by the researcher .
Titanium metal powder and the bronze colored , metallic titanium(II) oxide , TiO , could also be tried as the titanium reducing agents , replacing Ti2O3 in the above experiments . Nickel metal powder might be able to reduce Ti(IV) to Ti(III) as follows :
½x La2O3 + (1-x) SrO + TiO2 + ½x Ni0 + (1 ½x) NiO ------ [heat , nitrogen or argon atmosphere , or graphite blanket] -------> LaxSr1-xTiNiO4 ; x = 0 to 1 .
The Ti(III,IV) system is chemically reducing and would be redox-incompatible with the mildly oxidizing La2CuO4 matrix .
Vanadium , Niobium , and Tantalum Dioxides with La2CuO4 and La2NiO4
The covalent-metallic layers for the Group 5 (VB) elements vanadium , niobium , and tantalum would be derived from their tetravalent MO2 oxides . As discussed in the Ilmenites web page , VO2 and NbO2 are known to be metallic and electrically conductive (TaO2 probably is too , but I couldn't find any information about it) . Vanadium(V) is moderately oxidizing : V5+ + e- -------> V4+ ; E0ox = 0.957 V . This suggests that the Group 5 MO2 oxides are redox-compatible with both La2CuO4 and La2CuO4 , since Cu2+ e- -------> Cu3+ ; E0ox = 2.4 V , and Ni2+ e- -------> Ni3+ ; E0ox = 1.17 V .
As VO2 and NbO2 are commercially available (eg. Alfa-Aesar) , the most straightforward synthesis of their corresponding heterostructures with La2CuO4 and La2NiO4 would be the following method :
3 SrO + VO2 + CuO -------- (mix , heat , etc. , N2 or Ar atm) -------> Sr3VCuO6 [metallic , vanadium (IV) substrate] ;
2 SrO + ½ K2CO3 + ½ V2O5 + CuO -------- (mix , heat , etc. , flowing N2 or Ar atm) -------> Sr2KVCuO6 + ½ CO2 (g) [insulator , vanadium(V) dopant] ; then ,
x Sr3VCuO6 + (1-x) Sr2KVCuO6 -------- (mix , heat , etc. , N2 or Ar atm) ------> Sr2+xK1-xVCuO6 ; x = 0 to 1 .
BaM compounds based on modified La2CuO4 will have the general formula Ba3MCuO6 (Ba can be replaced in most cases by Sr or even Ca) . They can be thought of as the combination of the metallic perovskite BaMO3 and the AFM induction component Ba2CuO3 : BaMO3 + Ba2CuO3 ------> Ba3MCuO6 . In BaMNiO4 the Ni2+ cations are ionically bonded to the oxide anions ; single sheets of Ni2+O2- from the rocksalt NiO might theoretically be intercalated between the BaMO3 metallic layers . Copper(II) oxide has the unusual tenorite structure (GIF image , 38 KB) consisting of layers of CuO2 ribbons with coordinate covalent bonds between the Cu2+ cations and oxide anions . Unlike the NiO in BaMNiO4 , rocksalt-like sheets of CuO can't be intercalated between the BaMO3 metallic layers in the new heterostructure . A copper(II) oxide derivative with flat CuO sheets must be found to act as the AFM induction layer in the copper-based BaM compounds .
SrCuO2 is known to be moderately AFM [Arjomand and Machin(2)] ; it has flattened CuO2 sheets with intercalated Sr2+ cations :
However , SrCuO2 lacks axial CuO bonds through which AFM induction via electron superexchange into the metallic layer is transmitted , so it would be unsuitable as the AFM layer in the heterostructure . To obtain the necessary axial CuO bonds an additional molar equivalent of SrO must be added to the SrCuO2 to obtain Sr2CuO3 , which has six-coordinate tetragonal copper(II) in the CuO3 . Combination of alternating layers of the metallic SrMO3 and the AFM Sr2CuO3 results in the copper-based BaM formula Sr3MCuO6 , whose possible crystal structure is presented in the following sketch :
The axial CuO bonds should be elongated relative to the planar CuO bonds (not shown in the sketch) . In such copper oxide compounds the six-coordinate Cu(II) is affected by Jahn-Teller distortion , caused by the presence of the copper 3d9 singlet valence electron in the stereochemically prominent dz2 orbital . 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 these hypothetical Sr3MCuO6 compounds , as it does in La2CuO4 .
Tantalum dioxide might be prepared in situ by the reproportionation of Ta0 and Ta2O5 :
1/5 Ta0 + 2/5 Ta2O5 -------> TaO2 ; thus ,
SrO + 1/5 Ta0 + 2/5 Ta2O5 + NiO -------- (N2 or Ar atm) -------> SrTaNiO4 ;
½ Na2CO3 + ½ Ta2O5 + NiO -------- (flowing N2 or Ar atm) -------> NaTaNiO4 + ½ CO2 (g) ;
x SrTaNiO4 + (1-x) NaTaNiO4 -------- (N2 or Ar atm) -------> SrxNa1-xTaNiO4 ; x = 0 to 1 .
While these composites are mildly oxidizing in nature , I would nevertheless recommend the use of an inert atmosphere of pure , dry nitrogen or argon , or a graphite blanket , in their preparation and processing (annealing) as a standard precautionary measure . At the same time , the critical AFM induction component NiO will also be protected against oxidation and magnetic degradation .
Chromium , Molybdenum , and Tungsten Dioxides with La2CuO4 and La2NiO4
Chromium , molybdenum , and tungsten are among the ten elements (M = Ti , Zr , V , Nb , Ta , Cr , Mo , W , Re , and U) which form perovskite (or related) bronzes . The bronzes are brightly coloured solids with a metallic luster , and generally have high electrical conductivities at elevated levels of the doping component (often an Alkali metal) . For example , the Na0.5WO3 bronze has an ambient electrical conductivity of around 18,000 ohm-1cm-1 , and those with higher doping levels of Ax > 0.8 can have conductivities of up to 70,000 ohm-1cm-1 (Shanks and co-workers , 1963) , comparable to many metallurgical metals .
All the conductive bronzes display Pauli paramagnetism , with no evidence of any ferro- or antiferromagnetism . Rubidium tungsten bronzes have the highest transition temperatures of the Alkali metal tungsten bronzes , with the Tc = 1.98 K for RbxWO3 (x = 0.270.29) reported in 1965 . An efficient synthesis of rubidium tungsten bronzes in a microwave furnace was described in 2008 by J. Guo and co-workers . Tetragonal K0.5MoO3 has an unusually high (for a bronze) Tc = 4.2 K (Sleight and co-workers , 1969) .
Combining the highly metallic bronzes with the powerfully AFM La2CuO4 and La2NiO4 in multilayerd heterostructures should result in superb HTS candidate materials . The cubic perovskite SrCrO3 might be combined with Sr2CuO3 to produce the corresponding BaM compound : SrCrO3 + Sr2CuO3 ------> Sr3CrCuO6 , ie. (Sr2+)3(Cr4+) [(Cu2+) (O2-)6] . The Cr(IV) component is a powerful oxidizer [1.48 V to Cr3+] and would oxidize Ni2+ to Ni3+ [ 1.17 V] ; however , it can't oxidize Cu2+ to Cu3+ ( 2.4 V) .
SrCrO3 is a somewhat controversial compound . It can be prepared only under high pressures , which has discouraged its more widespread study . It was first synthesized in a tetrahedral anvil press at 65 Kbar and 800 ºC by Chamberland in 1967 ; his product was a black , crystalline solid , metallic with Pauli paramagnetism . Here's the Valence Bond sketch of a possible electronic structure for the metallic , cubic form of SrCrO3 as prepared by Chamberland :
SrCrO3 has also been described as as a cubic , paramagnetic insulator (Zhou and co-workers , 2006) , and as both a cubic and tetragonal perovskite (Ortega-San-Martin and co-workers , 2007) . The following Valence Bond sketch could portray the electronic state of tetragonal SrCrO3 , which has four long and two short CrO bonds :
Castillo-Martínez and Alario-Franco (2007) studied the electrical resistivity of a sintered disc of SrCrO3 , and like the Attfield group (Ortega-San-Martin and co-workers) found it was a pseudometal (with a direct temperatureelectrical conductivity relationship) , as with semiconductors like silicon . However , Chamberland's resistivity measurements on a pure , single crystal of SrCrO3 clearly showed it was a true metal (with an inverse temperatureelectrical conductivity relationship) , like the common metallurgical metals :
(my thanks to the author and/or copyright holder of this sketch)
Chamberland comments (in his review of CrO2 , see immediately above in the sketch , p. 11) :
Many early reports of the electrical resistivity [of CrO2 ] were very questionable since the materials studied were either impure or the data were obtained only on powdered compacts ....... Most results on the powdered compacts indicated semiconducting behaviour with low activation energies .......
It's now accepted that pure CrO2 is a true metal , like the metallurgical metals (for example , see Chamberland's graph of the electrical resistivity of CrO2 in Figure 9 , p. 13 , of his review of the material cited immediately above) . Similarly , his electrical resistivity data on a pure , single crystal of SrCrO3 support the conclusion that cubic SrCrO3 is similarly a true metal , as the Valence Bond sketch of it above suggests . This is very important in our study of chromium(IV) oxide BaM compounds , since only true metal compounds may exhibit superconductivity under suitable conditions ; pseudometals never can , because as they are cooled down they become insulators .
In the proposed BaM compound Sr3CrCuO6 we aren't dealing with pure SrCrO3 , but rather with monolayers of the compound intercalated with Sr2CuO3 layers , the latter imposing their moderately strong AFM electron spin ordering pattern on the CrO metallic bond mobile , free electrons . Hopefully the CrO2 layers in the BaM compound will be electronically similar to those in both CrO2 itself and in cubic (not tetragonal !) SrCrO3 .
The Cr(IV) component of Sr3CrCuO6 might be obtained by reducing the Cr(VI) in a chromate compound such as SrCrO4 with finely divided copper metal powder :
SrCrO4 + 2 SrO + Cu0 -------- (mix , heat) -------> Sr3CrCuO6 .
The strongly oxidizing nature of Sr3CrCuO6 would make the use of a protective atmosphere of nitrogen or argon in its preparation superfluous .
This redox reaction should be feasible , as indicated by the following equations :
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 : The Cr reduction reaction is actually Cr(V) + e- ------> Cr(IV) , as I couldn't find the E0red value for the Cr(VI) / Cr(IV) couple . However , as is well known , Cr(VI) is a powerful oxidizing agent , so the E0red = 1.34 V value shown for Cr(VI) / Cr(IV) is probably about right .
The reaction of copper metal powder with strontium chromate is thus predicted to be thermodynamically spontaneous , and indeed quite exothermic once underway , although the added 2 SrO should help to moderate the energetic reaction somewhat .
Any researcher attempting this reaction (SrCrO4 + 2 SrO + Cu0) 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 , strontium chromate , and strontium oxide (ground thoroughly together) would be placed in an over-sized porcelain crucible , 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 yellow salts SrCrO4 and BaCrO4 are commercially available (eg. Alfa-Aesar) , or they could be prepared by the simple metathesis reaction of a soluble strontium salt such as SrCl2 with Na2CrO4 in water solution . They are somewhat insoluble in water (SrCrO4 , 1.06 g/l ; BaCrO4 , 0.0026 g/l) and would precipitate from the reaction mixture from which they could be filtered , washed , and dried . Barium chromate is actually a well-known bright yellow pigment in paints and anti-corrosion pastes , replacing to some extent the more toxic lead chromate , PbCrO4 , the famous Chrome Yellow pigment . It should be noted in this respect that chromium(VI) compounds are considered to be toxic and carcinogenic , and should therefore be handled carefully and observing all the necessary safety precautions .
Chromium(IV) is a metastable valence state ; the stable ones for chromium are the zerovalent one , for elementary chromium ; Cr(II) , mildly reducing ; Cr(III) , low energy ; and Cr(VI) , strongly oxidizing . A very exothermic SrCrO4 + Cu0 reaction might cause the chromium atoms to skip over the metastable Cr(IV) state and disproportionate into Cr(III) and Cr(VI) . As an alternate procedure , Sr3CrCuO6 might be synthesized by adding one equivalent of CuO to the reaction mixture of SrO (now 3.0 equiv.) and CrO2 used by Chamberland in his high pressure synthesis of SrCrO3 . Another possible method would involve the reproportionation of Cr0 and CrO3 to CrO2 , which would react in situ with the SrO and CuO components :
3 SrO + 1/3 Cr0 + 2/3 CrO3 + CuO -------- (mix , heat) -------> Sr3CrCuO6 ; or ,
7/3 SrO + 1/3 Cr0 + 2/3 SrCrO4 + CuO -------- (mix , heat) -------> Sr3CrCuO6 ; or ,
3 SrO + 1/3 Cr0 + 2/3 CuCrO4 [see the following] + 1/3 CuO ----- (mix , heat) -----> Sr3CrCuO6 .
Sr3CrCuO6 is expected to be naturally metallic ; it could nevertheless be doped in varying mole ratios by a corresponding Cr(V) BaM compound such as KCrCuO4 (+ 2 SrO) . This dopant material , which is also probably metallic , might be prepared from the hydride or borohydride reduction of copper(II) chromate :
CuCl2 (aq) + Na2CrO4 (aq) ----------> CuCrO4 (red-brown , insoluble ppt) + 2 NaCl (aq) ;
NaH + CuCrO4 ----------> NaCrCuO4 + ½ H2 (g) ; or ,
NaBH4 + CuCrO4 ----------> NaCrCuO4 + ½ B2H6 (g) + ½ H2 (g) .
Note that copper(II) would also be reduced by hydride and borohydride to Cu(I) and Cu(0) ; however , the Cr(IV) in CuCrO4 is by far the more powerful oxidizer and should be preferentially reduced by a single molar equivalent of hydride or borohydride .
The application of hydrides and borohydrides in the synthesis of various bronzes was proposed in another Chemexplore web page , from which I'll copy and paste as follows :
Here's a suggestion for a simple route to sodium tungsten bronzes , also involving the evolution of a gaseous by-product but an expendable one and using only relatively inexpensive reagents :
x NaH + WO3 ---- (warm in a stream of pure flowing nitrogen) ------> NaxWO3 + ½ x H2 (g) ; and ,
x NaBH4 + WO3 ---- (pure flowing nitrogen) ------> NaxWO3 + ½ x H2 (g) + ½ x B2H6 (g) .
Both sodium hydride and sodium borohydride are well-known reagents in organic chemistry , the former as a strong but non-nucleophilic base , and the latter as a mild reducer of numerous organic functional groups . The corresponding lithium and potassium compounds are also known and are commercially available , but are less frequently used than the sodium chemicals . Hydride is a remarkably strong reducing agent , and it should readily transfer one of its 1s2 valence electrons to the mildly-oxidizing W(VI) in the WO3 :
2 W(VI) + 2e- -------------> 2 W(V) ............ E0red = 0.26 V ;
2 H1- 2e- -------------> H2 (g) ............ E0ox = 2.23 V ;
Net : 2 W(VI) + 2 H1- -------------> 2 W(V) + H2 (g) ............ E0T = 2.49 V
The substantial E0T suggests that the NaHWO3 reaction could be strongly exoergic , so the two compounds should not be compressed into a pellet , but rather gently mixed together as loose powders . Caution : please take the appropriate safety precautions if attempting these reactions ! Use of eye protection (safety glasses , with goggles or a visor) is obligatory , and a safety screen and leather gloves are strongly advised . Only semi-micro quantities of the reagents (no more than a gram or two) should be combined in exploratory work , until the researcher is confident that the reaction if successful , of course can be safely scaled up to a preparative level .
The powerful base potassium hydride , KH , is now available in the form of a 50 : 50 blend with solid paraffin wax . In this form it is very stable and easy to manipulate , with no loss of its basic properties (PDF , 35 KB) . KHparaffin could be very useful in the synthesis of KxWO3 , where the paraffin might dilute down the KH , and so moderate the addition reaction .
Borohydride isn't as strong a reducer (1.24 V in alkaline aqueous media) as hydride anion , but NaBH4 is easier to handle than NaH (although it's somewhat hygroscopic , which can be a nuisance on those humid summer days) . Aldrich supplies NaH in a dry powder form (95% pure) , which is more convenient than the mineral oil dispersions of the Alkali metal hydrides commonly available commercially . The tungsten bronzes are chemically quite robust , so the reaction products can be thoroughly washed with water or even dilute mineral acid to remove excess alkali and water-soluble sodium tungstate impurities without harming the bronze .
The substrate Sr3CrCuO6 could then be doped with varying mole ratios x of the dopant NaCrCuO4 (+ 2 SrO) , with which it would likely form solid solutions containing the electronically active Cr(IV,V) :
x Sr3CrCuO6 + (1-x) NaCrCuO4 + (2-2x) SrO ---- (mix , heat) -----> Sr2+xNa1-xCrCuO6 ; x = 0 to 1 .
The Attfield group (Ortega-San-Martin and co-workers) successfully reproportionated Cr(III) and Cr(VI) from the reagents Cr2O3 and Sr3(CrO4)2 , respectively , in their synthesis of SrCrO3 .
SrMoO3 is a cubic perovskite with an ambient electrical conductivity of 12,788 ohm-1cm-1 (Brixner , 1960) for a bulk specimen , or 8547 ohm-1cm-1 (Wang and co-workers , 2001) , increasing to 16,949 ohm-1cm-1 close to Absolute Zero , for epitaxial films . Maekawa and co-workers (2005) determined the electrical resistivity of SrMoO3 (cut section of a sintered pellet) to be of an order of magnitude of 10-6 ohm-m, with a positive temperature dependence . This translates into an electrical conductivity of 10,000 ohm-1cm-1 with an inverse temperatureelectrical conductivity relationship , typical of a true metal . They also mentioned that SrMoO3 is Pauli paramagnetic . Its colour was described as beetroot purple, which changes (together with the temperature dependence of the resistivity) at about 800 K :
...... the color of the sample surface has changed after the [resistivity] measurement . It seems that the sample becomes oxidized above 800 K (p. 315) .
Nagai and co-workers (2005) reported a much higher ambient electrical conductivity value for pure SrMoO3 single crystals of 196,078 ohm-1cm-1 , a figure comparable to that of many metallurgical metals (and even higher than that for rhenium trioxide , at 149,300 ohm-1cm1 . The electrical conductivity of molybdenum metal is 182,815 ohm-1cm1 at 298 K by comparison . The bilayer MoO metallic bond , a pi XO [see the VB sketch for cubic SrCrO3 above] , is clearly evident in these conductivity figures) . It should therefore be an excellent metallic compound for inclusion in a BaM heterostructure .
Molybdenum(VI) is a moderately strong oxidizer [E0red = 0.700 V to molybdenum(V)] , and could thus be successfully combined with copper(0) [E0ox = 0.3419 V to Cu2+] and nickel(0) [E0ox = 0.257 V to Ni2+] to provide the corresponding BaM compounds :
SrCl2 (aq) + Na2MoO4 (aq) ----------> SrMoO4 (insoluble ppt) + 2 NaCl (aq) ;
SrMoO4 + 2 SrO + Cu0 -------- (mix , heat , etc. , N2 or Ar atm) -------> Sr3MoCuO6 ;
SrMoO4 + Ni0 -------- (mix , heat , etc. , N2 or Ar atm) -------> SrMoNiO4 .
SrMoO4 and BaMoO4 are commercially available (eg. Alfa-Aesar) , but they could also be prepared by the researcher by a simple metathesis reaction with the water-soluble sodium molybdate , as indicated .
In an interesting variation of this last reaction , the three primary reagents would be thoroughly ground together and heated in a porcelain or graphite crucible , or evacuated , sealed tube , until a solid state metathesis reaction occurred :
SrCl2 + Li2MoO4 + Ni0 -------- (mix , heat , etc. , N2 or Ar atm) -------> SrMoNiO4 + 2 LiCl .
The two salts , SrCl2 (m.p. 874 ºC) and Li2MoO4 (m.p. 702 ºC) , are commercially available (eg. Alfa-Aesar) in their anhydrous forms . Additional lithium chloride (m.p. 610 ºC) could be added to the mixture of reagents , to act as an ionic flux for the reaction (the intermediate SrMoO4 melts at 1040 ºC) . Extraction of the powdered product with methanol , in which LiCl is very soluble (423.6 g/l at 25 ºC) , would leave the insoluble SrMoNiO4 residue , avoiding any possible hydrolysis of it . The SrMoNiO4 would be dried , finely ground , pressed into a cylindrical pellet , and annealed under an inert atmosphere . The researcher should be alert to the possibility of growing very pure , fairly large single crystals of product in a molten ionic flux such as lithium chloride . Such crystals are very desireable for electrical conductivity testing .
The molybdenum(VI) in MoO3 (m.p. 802 ºC) is reduced by nickel metal powder to Mo(IV) in the following synthesis of SrMoNiO4 and BaMoNiO4 :
SrO + MoO3 + Ni0 ----- (mix , heat , etc. , N2 or Ar atm , or graphite blanket) ------> SrMoNiO4 ;
BaCO3 + MoO3 + Ni0 ----- (mix , heat , etc. , flowing N2 or Ar atm) ------> BaMoNiO4 + CO2 (g) .
The corresponding Mo(V) dopant NaMoNiO4 for the synthesis of Mo(IV,V) mixed-valent composites might be prepared by a NaH or NaBH4 reduction of NiMoO4 :
NiCl2 (aq) + Na2MoO4 (aq) ----------> NiMoO4 (insoluble ppt) + 2 NaCl (aq) ;
NaH + NiMoO4 ----------> NaMoNiO4 + ½ H2 (g) ; or ,
NaBH4 + NiMoO4 ----------> NaMoNiO4 + ½ B2H6 (g) + ½ H2 (g) ;
x SrMoNiO4 + (1-x) NaMoNiO4 ------ (mix , heat) -------> SrxNa1-xMoNiO4 ; x = 0 to 1 .
BaWO3 is also a metallic perovskite bronze . Conroy and Yokokawa state , The chemical and electrical properties [of the barium tungsten bronzes that they studied , BaxWO3 ; x = 0 to 0.13] are similar to [those of ] the alkali metal bronzes. Their sample of Ba0.12WO3 had a measured electrical conductivity of 6536 ohm-1cm-1 at 298 K , and ~ 6900 ohm-1cm-1 at 150 K .
Tungsten(VI) is a rather weak oxidizer [E0red = 0.26 V to tungsten(V)] , and would be unable to oxidize Cu0 [E0ox = 0.3419 V to Cu2+] . It could , however , oxidize nickel metal powder [E0ox = 0.257 V to Ni2+] , so the BaM compound SrWNiO4 might be synthesized by the following simple reactions :
SrCl2 (aq) + Na2WO4 (aq) ----------> SrWO4 (insoluble ppt , 1.4 g/l ) + 2 NaCl (aq) ;
SrWO4 + Ni0 -------- (mix , heat , etc. , N2 or Ar atm , or graphite blanket) -------> SrWNiO4 .
Note that tungstate , WO42- , is a discrete molecular anion (as are chromate , CrO42- , and molybdate , MoO42-) ; its tungsten atom is tetrahedrally coordinated by oxygen atoms . Thus , in the proposed reactions above involving its reduction by nickel metal powder , the reduced tungstate monomers must then polymerize into an inorganic polymer, the BaM compound with an infinite atomic lattice .
The common and relatively inexpensive tungsten trioxide (m.p. 1473 ºC) could be reduced by nickel metal powder to WO2 in the presence of SrO or BaO (the latter component derived from BaCO3) , in a simple shake-'n-bake reaction :
SrO + WO3 + Ni0 ----- (mix , heat , etc. , N2 or Ar atm , or graphite blanket) ------> SrWNiO4 ;
BaCO3 + WO3 + Ni0 ----- (mix , heat , etc. , flowing N2 or Ar atm) ------> BaWNiO4 + CO2 (g) .
Since MoO2 and WO2 are commercially available (eg. Alfa-Aesar) , molybdenum and tungsten BaM compounds might also be prepared by a direct route in which no valence changes are involved :
3 SrO + MoO2 + CuO -------- (mix , heat , etc. , N2 or Ar atm , graphite) -------> Sr3MoCuO6 ;
SrO + MoO2 + NiO -------- (mix , heat , etc. , N2 or Ar atm , graphite) -------> SrMoNiO4 ;
SrO + WO2 + NiO -------- (mix , heat , etc. , N2 or Ar atm , graphite) -------> SrWNiO4 .
Note that the putative (Sr2+)3(W4+)(Cu2+)O6 might electronically be (Sr2+)3(W5+)(Cu1+)O6 or even (Sr2+)3(W6+)(Cu0)O6 . Neither Cu1+ nor Cu0 could provide the required AFM induction to the metallic WO2 layers , which would never become superconducting .
At the beginning of this section I mentioned the highly conducting sodium tungsten bronze Na0.8WO3 , with an ambient electrical conductivity of ~ 70,000 ohm-1cm-1 . It's a Robin-Day Class II mixed-valent compound whose valence-counting formula can be written as (Na1+)0.8(W5+)0.8(W6+)0.2(O)36- . Na0.8WO3 might be combined with NiO to obtain the BaM compound Na0.8WNiO4 :
0.4 Na2WO4 + 0.6 WO3 + 0.4 Ni0 + 0.6 NiO ------ (mix , heat , etc. , N2 or Ar atm , graphite) ------> Na0.8WNiO4 ; more generally ,
½x Na2WO4 + (1 ½x) WO3 + ½x Ni0 + (1 ½x) NiO ------ (mix , heat , etc. , N2 or Ar atm , graphite) ------> NaxWNiO4 ; x = 0 to 1 ; alternately ,
0.4 Na2CO3 + WO3 + 0.4 Ni0 + 0.6 NiO ------ (heat , flowing N2 or Ar atm) ------> Na0.8WNiO4 + 0.4 CO2 (g) ; more generally ,
½x Na2CO3 + WO3 + ½x Ni0 + (1 ½x) NiO ------ (heat , flowing N2 or Ar atm) ------> NaxWNiO4 + ½x CO2 (g) .
Most (80 mole per cent) of the tungsten(VI) in the anhydrous sodium tungstate (m.p. 695 ºC) and tungsten trioxide reagents would be reduced by the nickel to tungsten(V) : 2 W(VI) + Ni(0) ------> 2 W(V) + Ni(II) , E0T = 0.517 V , which is moderately exoergic . The composite product , Na0.8WNiO4 , would combine both the high electrical conductivity of the sodium tungsten bronzes with the powerful AFM induction in the ionic Ni2+O2- layers .
Pure , anhydrous K2WO4 , Rb2WO4 , K2MoO4 , and Rb2MoO4 are commercially available , eg. Alfa-Aesar , and could be used in analogous reactions . Recall that the rubidium tungsten bronzes have the highest transition temperatures of any of the tungsten bronzes , and that tetragonal K0.5MoO3 also has a relatively high Tc = 4.2 K . The alkali molybdenum bronzes could also be intercalated with CuO3 layers : K0.5MoO3 + Ba2CuO3 -------> K0.5Ba2MoCuO6 . For example ,
¼ K2MoO4 (m.p. 919 ºC) + ¾ MoO3 (m.p. 802 ºC) + 2 BaCO3 + ¼ Cu0 + ¾ CuO ------ (heat , flowing N2 or Ar atm) ------> K0.5Ba2MoCuO6 + 2 CO2 (g) ; alternately ,
¼ K2CO3 + MoO3 + 2 BaCO3 + ¼ Cu0 + ¾ CuO ------ (heat , flowing N2 or Ar atm) ------> K0.5Ba2MoCuO6 + 2¼ CO2 (g) .
Obviously the combination of the Alkali metal bronzes with NiO and CuO2 AFM induction layers into many new heterostructures has a wide scope for future investigation .
In the chromium , molybdenum , and tungsten BaM compounds the exceptionally high electrical conductivities of the MO2 layers , combined with strong AFM induction from the NiO and CuO2 layers , should result in the production of multilayered heterostructures with remarkably high superconductor transition temperatures , undoubtedly well above 100 K . Group 6 BaM compounds have an excellent potential to be high performance HTS materials .
Manganese and Rhenium Dioxides with La2CuO4 and La2NiO4
In stark contrast to the Group 6 BaM compounds , those of the Group 7 manganese(IV) and rhenium(IV) would be only poorly metallic and probably low temperature superconductors . Manganese dioxide is a semiconductor-like pseudometal , with an ambient electrical conductivity of ~ 20 ohm-1cm-1 (Bowen) . BaMnO3 and SrMnO3 are antiferromagnetic insulators with T = 260 K and 220 K respectively (Adamo and co-workers , 2008 ; Korneta and co-workers , 2010) . The latter manganate has the perovskite structure , but remarkably it can crystallize in either a high temperature cubic form or a low temperature hexagonal polymorph (Søndenå and co-workers , 2006) . BaMnO3 exists only in the hexagonal perovskite-related form (Korneta and co-workers) because the Ba2+ cation is over-sized for the surrounding MnO3 cage (the Goldschmidt tolerance ratio t for BaMnO3 is t = 1.146) . CaMnO3 is a cubic symmetry perovskite , with more comfortable fitting (t = 1.036) Ca2+ cations in the surrounding MnO3 lattice . SrMnO3 has an intermediate t = 1.077 ; CdMnO3 would have t = 1.024 ; ZnMnO3 might have t = 0.948 [guesstimate] , in the middle of the cubic symmetry range (t ~ 0.901.00) for perovskites .
A cubic symmetry Mn(IV) BaM compound , say Ca3MnCuO6 [ie. (Ca2+)3(Mn4+)(Cu2+O6)] , would likely be an insulator or at best a poor semiconductor . It could be electronically activated by doping with a corresponding Mn(V) analogue such as NaMnCuO4 (+ 2 CaO) to create a Mn(IV,V) Robin-Day Class II mixed-valent compound :
(2+x) CaO + ½(1-x) Na2CO3 + MnO2 + CuO + ¼(1-x) O2 (g) ------- mix , heat , pure flowing oxygen atmosphere ---------> Ca2+xNa1-xMnCuO6 + ½(1-x) CO2 (g) ; x = 0 to 1 .
Manganese(IV) is a powerful oxidizer [E0red = 1.224 V to Mn2+] and would certainly oxidize nickel(II) [E0ox = 1.17 V to Ni3+] . Therefore La2NiO4 can't be used as the host matrix for any manganese(IV) BaM compounds .
Rhenium dioxide is a gray crystalline solid , fairly stable (dec. ~ 900 ºC) , and somewhat metallic (ambient s ~ 50 ohm-1cm-1 , Bowen) . Rhenium(IV) is a mild oxidizer [E0red = 0.2513 V to Re0] , so its BaM compounds within both La2CuO4 and La2NiO4 host structures are possible . The crystal ionic radius of Re(IV) , six-coordinate octahedral , per Shannon and Prewitt , is r = 0.63 Å , compared to the slightly smaller r = 0.53 Å for Mn(IV) . BaM rhenates might thus be quite similar to the manganates . Ca3ReCuO6 could be doped with the corresponding analogue Ca2NaReCuO6 to produce the electronically-active Re(IV,V) mixed-valent composites Cax+2Na1-xReCuO6 , as with the manganates ; similarly for CaReNiO4 , NaReNiO4 , and CaxNa1-xReNiO4 .
As to why the manganates are AFM insulators , I suspect the electronic condition of their manganese(IV) atoms , which are 3d3 electronically , might be somewhat like the chromium(IV) atoms in tetragonal SrCrO3 , whose sketch is shown above . However , no Jahn-Teller tetragonal distortion is observed for cubic SrMnO3 , so the manganese atom in this latter case may have hybridized its 3dz2 valence electron and orbital in a d2sp3 octahedral orbital , which is used for its MnO covalent bonds :
It's interesting to note in this regard that the early Transition metals tend to use all or most of their d orbitals and valence electrons in forming octahedral hybrid orbitals for MO covalent bonds ; thus , the d5s octahedral orbital is quite commonly created by them for this purpose . When that happens , any extra , leftover M valence electrons are located in the empty frontier n+1 py,z orbitals . In a perovskite these M py,z orbitals can usually overlap with the oxygen linking atoms' 2py,z orbitals with their electron pairs . These M and O p orbitals have the correct shape , symmetry , and orientation for overlapping continuously in the crystal lattice to form excellent MO pi XO metallic bonds . They are also of the bilayer variety , with rich populations of mobile , free electrons above EF : perfect for superconductivity , and for HTS if combined with a suitably strong AFM induction from a neighbouring AFM layer .
On the other hand , in the later Transition metals , more and more p character gradually creeps into the octahedral hybrid orbitals , with less and less d character , as the M element moves toward the p block of elements . Then , the d2sp3 (inner) , sp3d2 (outer) , and d3p3 hybrid orbitals are used by the M atoms for forming MO covalent bonds . When that occurs , the unused d orbital valence electrons can't be promoted to the n+1 p orbitals ; they stay in the original d orbitals . However , d orbitals can't form nodeless XOs with the oxygen p orbitals ; they have the wrong shape and symmetry for that . They might be able to form nodal d-p MOs , but only semiconductor or insulator behaviour can result from any sort of nodal MO (as in silicon , for example) . When M valence electrons are stranded in their original d orbitals in MO compounds , they are often pinned and the material is an insulator or poor semiconductor . This situation seems to have occurred in the manganates such as in cubic SrMnO3 , and maybe to a somewhat lesser extent in the rhenates (and inexplicably in tetragonal SrCrO3) .
Iron(IV) and Lead(IV) Dioxides in La2CuO4 BaM Compounds
SrFeO3 has an appreciable ambient electrical conductivity (~ 750 ohm-1cm-1 per H.K. Bowen , Figure 11 , p. 307 ; ~ 1000 ohm-1cm-1 per MacChesney , Sherwood , and Potter , for stoichiometric material) . A copper-based iron(IV) BaM compound might therefore be feasible . Fe(IV) could be a strong enough oxidizer to oxidize Ni(II) to Ni(III) [E0ox = 1.17 V to Ni3+] , so modified La2NiO4 iron(IV) analogues are unlikely prospects . The copper-based Fe(IV) materials would resemble the manganates immediately above :
(2+x) SrO + ½(1-x) Na2CO3 + ½ Fe2O3 + CuO + (½¼x) O2 (g) ------- mix , heat , pure flowing oxygen atmosphere ---------> Sr2+xNa1-xFeCuO6 + ½(1-x) CO2 (g) ; x = 0 to 1 .
Shannon and Bierstedt (1970) described the high temperature and hydrothermal syntheses of BaPbO3 , which is a metallic perovskite with a substantial electrical conductivity (s = 3448 ohm-1cm-1 at 296 K ; 13,514 ohm-1cm-1 in liquid helium at 4.2 K) . Lead(IV) is too strong an oxidizer [E0red = 1.455 V to Pb2+] for Ni(II) [E0ox = 1.17 V to Ni3+] , so only the copper-based BaM lead(IV) compounds would be feasible . They might be prepared by Shannon and Bierstedt's procedures for BaPbO3 , with the addition of BaO + CuO ; for example ,
3 BaCO3 + PbO + CuO + ½ O2 (g) ----- (heat , pure flowing oxygen atmosphere) -----> Ba3PbCuO6 + 3 CO2 (g) .
As with BaPbO3 , Ba3PbCuO6 might be naturally metallic ; however , it could also be doped with the hypothetical lead(III) dopant Ba2LaPbCuO6 , generated in situ in the reaction mixture :
(2+x) BaCO3 + ½ (1-x) La2O3 + PbO + CuO + ½ O2 (g) ----- (heat , pure flowing oxygen atmosphere) -----> Ba2+xLa1-xPbCuO6 + (1+x) CO2 (g) ; x = 0 to 1 .
These latter composites with the La3+ dopant could be metallic and possibly superconducting . Drozd and co-workers (2004) examined several Pb(III,IV) mixed-valent perovskites in the series Sr1-xLaxPbO3 (x = 0 , 0.05 , 0.10 , 0.15 , and 0.20) , claiming that , Samples of Sr0.9La0.1PbO3 incorporate small superconducting domains (p. 7) . Pure , undoped SrPbO3 is a genuine insulator with strong manifestations of current carrier localization at low temperatures. In fact , all of their lanthanum-doped samples had unmistakeable semiconducting properties , so I find it surprising that any traces of superconductivity were detected in them . Hadjarab and co-workers have also confirmed that SrPbO3 is a semiconductor , quite unlike BaPbO3 . Possibly the smaller Sr2+ cation , because of its more concentrated positive charge than that of the larger Ba2+ cation , is attracting electron density away from the orbitals that would otherwise be forming the metallic bond in the lattice . In any case , SrPbO3 appears to be unsuitable for use in any Pb(IV) BaM compounds .
Lee and co-workers failed to produce a new compound with alternating PbO and CuO layers when sintering BaPbO3 and YBCO together at 9001000 ºC . 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) .
A better technique , which is recommended for the synthesis of all the BaM heterostructures discussed in this web page , is to start with primary materials such as BaCO3 , PbO , and CuO , for example .
Another problem that can arise with lead compounds such as PbO is their volatility at high temperatures , as was noted in a technical brochure from NTT Laboratories , New Superconducting Lead Cuprates Prepared by Molecular Beam Epitaxy [PDF , 38 KB] . To avoid contaminating the laboratory with toxic lead emissions , a chemie douce synthesis of Ba3PbCuO6 and Ba2+xLa1-xPbCuO6 might be preferable . For example , a water solution of Ba(NO3)2 , La(NO3)3 , Pb(NO3)2 , and Cu(NO3)2 would be poured into a water solution of NaOH , with vigorous stirring . The precipitated product would be filtered , washed , and dried at 110 ºC . Shannon and Bierstedt's hydrothermal method of heating the intermediate with potassium chlorate in a sealed gold tube under high pressure (700 ºC at 3000 atmospheres for 24 hrs) would then oxidize the Pb(II) to Pb(IV) as the KClO3 decomposes to KCl and O2 . This procedure yielded black cubes of BaPbO3 , about 1 mm per side , crystallizing on the sides of the tube . Hopefully a similar result would be obtained for Ba3PbCuO6 and Ba2+xLa1-xPbCuO6 , with zero toxic lead emissions into the laboratory .
Mixed-Valent Bi(III,V) in La2CuO4 BaM Compounds
It should be possible to design and synthesize bismuth-based BaM compounds within the La2CuO4 matrix : (La3+ Bi3+)(Cu2+)(O2-)4 , (Ba2+ Bi4+)(Cu2+)(O2-)4 , and (K1+ Bi5+)(Cu2+)(O2-)4 . The first cuprate , LaBiCuO4 , is inadvisable , since its Bi(III) has a 6s2 inert pair that might prove difficult reproportionate into a metallic bond . The second bismuth cuprate , BaBiCuO4 , might also be troublesome . Bismuth(IV) compounds , while formally having a 6s1 valence electron , in practice seem to be adversely affected by disproportionation into Bi(III+V) that localizes these electrons in inert pairs and results in semiconducting behaviour in the material . For example , Sleight , Gillson , and Bierstedt (1975) grew bronze colored crystals of BaBiO3 hydrothermally , finding ,
Semiconducting behaviour is indicated over this entire range [4.2970 K] with a room temperature resistivity of about 10 ohm-cm ..... (p. 27) .
Bi(III+V) disproportionation can be avoided in Robin-Day Class II mixed-valent compounds with a NIOS (non-integral oxidation state , ie. fractional overall valence state) . Cava and co-workers (1988) synthesized an excellent superconductor , Ba0.6K0.4BiO3 (Tc = 29.8 K) , with electronically-active Bi(IV,V) . The remarkably high transition temperature of this compound a cubic perovskite suggests it has a bilayer BiO metallic bond . All it lacks for an improved HTS performance is an intercalated AFM layer to suppress its Pauli paramagnetism at higher temperatures . This aim might be realized in closely related Bi(IV,V) BaM heterostructures .
The corresponding BaM cuprate would be comprised of alternating Ba0.6K0.4BiO3 metallic and Ba2CuO3 AFM induction layers : Ba0.6K0.4BiO3 + Ba2CuO3 ---------> Ba2.6K0.4BiCuO6 . The Bi(V) component of Ba0.6K0.4BiO3 is a very powerful oxidizer [E0red = 1.769 V to Bi(III)] , and indeed Cava's research team used strongly oxidizing conditions , including potassium superoxide , KO2 , to remove some of the 6s2 inert pairs from the Bi(III) atoms in the Bi2O3 reagent . The remaining 6s2 inert pairs resonate in the BiO metallic bond as the mobile , free electrons above EF in the BiO3 lattice . The mixed-valent Bi(III,V) in Ba0.6K0.4BiO3 has a NIOS valence of +4.4 .
The following chemical equation combines Cava's synthesis procedure together with two additional equivalents of BaO and one of CuO for the Ba2CuO3 layers :
2.6 BaO + 0.4 KO2 + ½ Bi2O3 + CuO ------- (1) 675 ºC , 3 days
------- (2) anneal in pure O2 , 475 ºC --------> Ba2.6K0.4BiCuO6 .
Note that while the superoxide can oxidize Bi(III) to Bi(V) [1.769 V] , it's incapable of oxidizing Cu2+ to Cu3+ [2.4 V] . Both the superoxide and Bi(V) could oxidize Ni2+ to Ni3+ [1.17 V] , so Bi(IV,V) formulations are redox-incompatible with La2NiO4 .
Strontium analogues of Ba0.6K0.4BiO3 have been prepared , but disappointingly have much lower transition temperatures than Cava's compound . For example , Sr1-xKxBiO3 (x = 0.450.6 , Tc ~ 12 K) and Sr1-xRbxBiO3 (x = 0.5 , Tc ~ 13 K) were synthesized and studied in 1997 . The strontium BaM compound Sr2.6K0.4BiCuO6 would probably be accessible by an analogous procedure to that outlined above for Ba2.6K0.4BiCuO6 , but might not have as high a transition temperature as it .
The well-known heterostructure BSCCO-2212 , Sr2Bi2Ca2Cu3O10+x (Tc ~ 110 K ; Maeda and co-workers , 1988) , has metallic layers of Sr2Bi2O5+x with mixed-valent Bi(III,V) , alternating with AFM induction layers of Ca2Cu3O5 with homovalent copper(II) :
Ba2.6K0.4BiCuO6 should be at least as successful as BSCCO-2212 as a superconductor , and could conceivably have a transition temperature substantially above 100 K . Its successful synthesis and exceptional physical properties would provide additional supporting evidence for the AFM induction model of high temperature superconductivity .
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. Vanýsek (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) .
French researchers : M. Foëx , A. Mancheron , and M. Liné , Sur une Combinaison du Sesquioxyde de Lanthane avec le Protoxyde de Nickel, Comptes Rendus 250 (18) , pp. 3027-3028 (1960) .
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) .
Ruddlesden-Popper phases : W. Singkha and S. Kuharuangrong , Synthesis and Physical Properties of La3-xSrxNi2O7±g Ruddlesden-Popper Phase, J. Metals Mater. Min. 18 (2) , pp. 77-82 (2008) [PDF , 1013 KB . Note : this PDF file can be opened only with Adobe Acrobat Reader v. 6 or later . If desired , this application can be downloaded for free from Oldversion.com] ; I.B. Sharma and B. Singh , Solid State Chemistry of Ruddlesden-Popper Type Complex Oxides, Bull. Mater. Sci. 21 (5) , pp. 363-374 (1998) [HTML (download page) ; its PDF is 1416 KB] ; G.M. Sarjeant et al. , Synthesis and Structure of LaSr2CuTiO6.5 : A New Oxygen-Deficient Ruddlesden-Popper Phase, Chem. Mater. 8 (12) , pp. 2792-2798 (1996) [PDF , 260 KB] ; R.E. Schaak and T.E. Mallouk , Prying Apart Ruddlesden-Popper Phases : Exfoliation into Sheets and Nanotubes for Assembly of Perovskite Thin Films, Chem. Mater. 12 (11) , pp. 3427-3434 (2000) [PDF , 409 KB] ; see also Revisiting the SrCr(IV)O System ....., in Castillo-Martínez and Alario-Franco below , where the authors synthesized and studied several Cr(IV) Ruddlesden-Popper compounds , eg. Sr3Cr2O7 (= 2 SrCrO3 perovskites + 1 SrO rocksalt) . Crystal structures of their R-P phases are illustrated in Fig. 1 , p. 565 .
Cox : P.A. Cox , Transition Metal Oxides , An Introduction to Their Electronic Structure and Properties , Clarendon Press , Oxford (UK) , 1995 ; 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. Emeléus 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 .
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 .
Robin-Day Class II : M.B. Robin and P. Day , Mixed Valence Chemistry A Survey and Classification, Adv. Inorg. Chem. Radiochem. 10 , pp. 247-422 , H.J. Emeléus 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 Composés à 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 the web page , New Solar Cells from Mixed-Valent Compounds, with various examples of each class , and illustrated with sketches of the examples .
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) .
Kestigan and Ward : M. Kestigan and R. Ward , The Preparation of Lanthanum Titanium Oxide , LaTiO3, J. Amer. Chem. Soc. 76 (23) , p. 6027 (1954) .
rocksalt-like sheets of CuO : Grant has speculated about the properties of the hypothetical cubic rocksalt copper(II) oxide : P.M. Grant , Electronic Properties of Rocksalt Copper Monoxide : A Proxy Structure for High Temperature Superconductivity, J. Phys. : Conf. Ser. 129 (1) , 012042 , 8 pp. (2008) [PDF , 1242 KB] . He concluded that such a material would be a metallic solid . This research paper was downloaded from Paul Grant's website , which provides free PDF copies of many of his scientific publications .
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 ; Table 3 , p. 1062 . The crystal structures of many cuprate compounds are illustrated in the review article by H. Müller-Buschbaum , The Crystal Chemistry of Copper Oxometallates, Angew. Chem. Internat. Ed. Engl. 30 (7) , pp. 723-744 (1991) .
bronzes : P.G. Dickens and M.S. Whittingham , The Tungsten Bronzes and Related Compounds, Quart. Rev. 22 (1) , pp. 30-44 (1968) .
Shanks and co-workers : 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 ; M.J. Sienko , Electric and Magnetic Properties of the Tungsten and Vanadium Bronzes, Ch. 21 , pp. 224-236 in Nonstoichiometric Compounds ; 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 [reprinted by R.E. Krieger , Huntington , NY] .
Rubidium tungsten bronzes : A.R. Sweedler , Ch. J. Raub , and B.T. Matthias , Superconductivity of the Alkali Tungsten Bronzes, Phys. Lett. 15 (2) , pp. 108-109 (1965) ; R.K. Stanley , R.C. Morris , and W.G. Moulton , Conduction Properties of the Hexagonal Tungsten Bronze , RbxWO3, Phys. Rev. B20 (5) , pp. 1903-1914 (1979) .
J. Guo and co-workers : J. Guo et al. , Crystal Structure and Superconductivity of Rubidium Tungsten Bronzes RbxWO3 Prepared by a Hybrid Microwave Method, Mater. Res. Bull. 43 (4) , pp. 779-786 (2008) . They reported a maximum T = 5.3 K (onset) for Rb0.14WO3 ; J. Guo et al. , A Green Route for Microwave Synthesis of Sodium Tungsten Bronzes NaxWO3 (0<x<1), J. Solid State Chem. 178 (1) , pp. 58-63 (2005) .
Sleight and co-workers : A.W. Sleight , T.A. Bither , and P.E. Bierstedt , Superconducting Oxides of Rhenium and Molybdenum with Tungsten Bronze Type Structures, Solid State Commun. 7 (2) , pp. 299-300 (1969) .
Chamberland : B.L. Chamberland , Preparation and Properties of SrCrO3, Solid State Commun. 5 (8) , pp. 663-666 (1967) ; J.F. Weiher , B.L. Chamberland , and J.L. Gillson , Magnetic and Electrical Transport Properties of CaCrO3, J. Solid State Chem. 3 (4) , pp. 529-532 (1971) ; B.L. Chamberland and C.W. Moeller , A Study on the PbCrO3 Perovskite, J. Solid State Chem. 5 (1) , pp. 39-41 (1972) .
Zhou and co-workers : J.-S. Zhou et al. , Anomalous Electronic State in CaCrO3 and SrCrO3, Phys. Rev. Lett. 96 (4) , 046408 , 4 pp. (2006) ; A.C. Komarek et al. , CaCrO3 : An Anomalous Antiferromagnetic Metallic Oxide, Phys. Rev. Lett. 101 (16) , 167204 , 4 pp. (2008) .
Ortega-San-Martin and co-workers : L. Ortega-San-Martin et al. , Microstrain Sensitivity of Orbital and Electronic Phase Separation in SrCrO3, Phys. Rev. Lett. 99 (25) , 255701 , 4 pp. (2007) ; see also : Y. Qian et al. , The Electronic Structure of a Weakly Correlated Antiferromagnetic Metal , SrCrO3 : First-Principles Calculations, New J. Phys. 13 (5) , 053002 , 20 pp. (2011) [preprint available from ArXiv.org , PDF , 244 KB] .
Castillo-Martínez and Alario-Franco : E. Castillo-Martínez and M.Á. Alario-Franco , Revisiting the SrCr(IV)O System at High Pressure and Temperature with Special Reference to Sr3Cr2O7, Solid State Sci. 9 (7) , pp. 564-573 (2007) [PDF , 1998 KB] ; Fig. 10 , p. 571 . Of related interest : M.Á. Alario-Franco , E. Castillo-Martínez , and Á.M. Arévalo-López , The A(II)Cr(IV)O3 (A=Sr, Ca, Pb) Simple Perovskites . Structure and Properties : Magnetic Structure of CaCrO3, High Press. Res. 29 (2) , pp. 254-260 (2009) ; E. Castillo-Martínez et al. , Increasing the Structural Complexity of Chromium(IV) Oxides by High-Pressure and High-Temperature Reactions of CrO2, Inorg. Chem. 47 (19) , pp. 8526-8542 (2008) .
Chamberland's resistivity measurements : 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) ; Figure 16 , p. 22 .
monolayers : In a fascinating paper Scherwitzl and co-workers describe the correlation between the electrical conductivity properties of the metallic compound LaNiO3 with the thickness (in terms of unit cells, u.c.) of its layers deposited on an inert SrTiO3 surface by magnetron sputtering . They found that there was a quantum of resistance in two dimensions below which LaNiO3 ceased being a True Metal [inverse temperatureelectrical conductivity relationship] and became a Pseudometal [semiconductor ; direct temperatureelectrical conductivity relationship] . This occurred abruptly in epitaxial LaNiO3 thin films between 5 and 6 u.c. , as shown in their ArXiv.org preprint , Fig. 1 , p. 2 :
(my thanks to the author and/or copyright holder of this sketch)
R. Scherwitzl et al. , Metal-Insulator Transition in Ultrathin LaNiO3 Films, Phys. Rev. Lett. 106 (24) , 246403 , 4 pp. (2011) ; preprint available from ArXiv.org [PDF , 267 KB] .
Obviously this MI transition is absent in bulk samples of LaNiO3 ; but would it be present in a BaM heterostructure containing monolayers of LaNiO3 as the covalent-metallic component , such as in the composite Sr2LaNiCuO6 , which is (Sr2+ Sr2+ La3+)(Ni3+ Cu2+)(O2-)6 , ie. Sr2CuO3 + LaNiO3 ?
2 SrO + ½ La2O3 + NiO + CuO + ¼ O2 (g) -------- (mix , heat , pure oxygen atm) --------> Sr2LaNiCuO6 ; more generally , with n layers of metallic LaNiO3 :
2 SrO + ½ n La2O3 + n NiO + CuO + ¼ n O2 (g) ----- (mix , heat , pure oxygen atm) -----> Sr2LanNinCuO3n+3 ;
n could range between 1 and maybe as high as 30 , based on Scherwitzl's findings .
What about all the other metallic layers in the BaM compounds in general ? Would their electrical conductivities be significantly increased by using thicker layers of them in the heterostructures ? I hope researchers investigate this very important aspect of HTS materials some day .
reagents in organic chemistry : L. F. Fieser and M. Fieser , Reagents for Organic Syntheses , vol. 1 , John Wiley , New York , 1967 . NaH , pp. 1075-1081 ; NaBH4 , pp. 1049-1055 .
Brixner : L.H. Brixner , X-ray Study and Electrical Properties of System BaxSr(1-x)MoO3, J. Nucl. Inorg. Chem. 14 (3-4) , pp. 225-230 (1960) .
Wang and co-workers : H.H. Wang et al. , Epitaxial Growth and Electric Characteristics of SrMoO3 Thin Films, J. Vac. Sci. Technol. A 19 (3) , pp. 930-933 (2001) [PDF , 264 KB] ; H.H. Wang et al. , Growth and Characterization of SrMoO3 Thin Films, J. Crystal Growth 226 (2-3) , pp. 261-266 (2001) [PDF , 283 KB] .
Maekawa and co-workers : T. Maekawa et al. , Thermal and Electrical Properties of Perovskite-Type Strontium Molybdate, J. Alloys & Cmpds. 390 (1-2) , pp. 314-317 (2005) . See also N. Shirakawa and S.I. Ikeda , The Synthesis and Basic Physical Properties of a Layered Molybdenum Perovskite Sr2MoO4, Physica C 364-365 , pp. 309-312 (2001) . This latter material was a metallic solid but failed to become superconducting near Absolute Zero .
Nagai and co-workers : I Nagai et al. , Highest Conductivity Oxide SrMoO3 Grown by a Floating-Zone Method Under Ultralow Oxygen Partial Pressure, Appl. Phys. Lett. 87 (2) , 024105 , 3 pp. (2005) .
solid state metathesis reaction : J.B. Wiley and R.B. Kaner , Rapid Solid-State Precursor Synthesis of Materials, Science 255 (5048) , pp. 1093-1097 (1992) ; R.E. Treece , G.S. Macala , and R.B. Kaner , Rapid Synthesis of GaP and GaAs from Solid-State Precursors, Chem. Mater. 4 (1) , pp. 9-11 (1992) ; R.B. Kaner et al. , Rapid Solid-State Methathesis of Refractory Materials, U.S. Patent 5110768 , 8 pp. (May 5 , 1992) [PDF , 368 KB] ; R.B. Kaner , C.H. Wallace , and T.K. Reynolds , Instantaneous Synthesis of Refractory Nitrides from Solid Precursors, U.S. Patent 6096282 , 11 pp. (August 1 , 2000) [PDF , 127 KB] ; R.B. Kaner and C.H. Wallace , Process for Rapid Solid-State Formation of Refractory Nitrides, U.S. Patent 6120748 , 8 pp. (September 19 , 2000) [PDF , 89 KB] . Note : these three patent files can be opened only with Adobe Acrobat Reader v. 6 or later .
Conroy and Yokokawa : L.E. Conroy and T. Yokokawa , The Preparation and Properties of a Barium Tungsten Bronze, Inorg. Chem. 4 (7) , pp. 994-996 (1965) ; L.E. Conroy and G. Podolsky , Preparation of Tungsten Bronzes from Metal Halides, Inorg. Chem. 7 (3) , pp. 614-615 (1968) .
Bowen : 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 .
Adamo and co-workers : C. Adamo et al. , Electrical and Magnetic Properties of (SrMnO3)n / (LaMnO3)2n Superlattices, Appl. Phys. Lett. 92 (11) , 112508 , 3 pp. (2008) [PDF , 434 KB] .
Korneta and co-workers : O.B. Korneta et al. , Giant Magnetoelectric Effect in Antiferromagnetic BaMnO3-d and Its Derivatives, ArXiv.org , 21 pp. (November 10 , 2010) [PDF , 1389 KB] .
Søndenå and co-workers : R. Søndenå , P. Ravindran , and S. Stølen , Electronic Structure and Magnetic Properties of Cubic and Hexagonal SrMnO3, Phys. Rev. B 74 (14) , 144102 , 12 pp. (2006) .
Goldschmidt tolerance ratio t : U. Müller , Inorganic Structural Chemistry , John Wiley , Chichester , UK , 1993 ; p. 200 ; A.F. Wells , Structural Inorganic Chemistry , third ed. , Clarendon Press , Oxford , UK , 1962 ; p. 497 . It's also mentioned in the WolfWiki Perovskite web page at http://wikis.lib.ncsu.edu/index.php/Perovskite . The Goldschmidt equation is :
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) .
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) .
Drozd and co-workers : V.A. Drozd et al. , Hidden Superconductivity in Solid Solutions Sr1-xLaxPbO3 Detected by Tunneling, ArXiv.org , 16 pp. (September 6 , 2004) [PDF , 183 KB] .
Hadjarab and co-workers : B. Hadjarab et al. , The Physical Properties of Oxygen-Deficient Perovskite SrPbO3-g, J. Phys. Condens. Matter 18 (37) , 8551 (2006) ; B. Hadjarab et al. , Physical Properties and Photoelectrochemical Characterization of SrPbO3, Phys. Status Solidi A 204 (7) , pp. 2369-2380 (2007) .
Lee and co-workers : T.G. Lee et al. , Synthesis of YBa2Cu3O7-dBaPbO3 Composites, J. Korean Phys. Soc. 31 (3) , pp. 410-413 (1997) [PDF , 2878 KB] .
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 , Cambridge University Press , Cambridge (UK) , 1997 ; p. 128-137 , ; Ohio State web page at http://www.chemistry.ohio-state.edu/~woodward/ch754/syn_cd.htm .
Sleight , Gillson , and Bierstedt : A.W. Sleight , J.L. Gillson , and P.E. Bierstedt , High-Temperature Superconductivity in the BaPb1-xBixO3 System, Solid State Commun. 17 (1) , pp. 27-28 (1975) .
Cava and co-workers : R.J. Cava et al. , Superconductivity Near 30 K Without Copper : The Ba0.6K0.4BiO3 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 BaKBiO System, Phys. Rev. B 37 (7) , pp. 3745-3746 (1988) ; J.J. Krajewski , Synthesis of Superconducting Oxides, pp. 192-201 in Inorg. Synth. 30 , Nonmolecular Solids , D.W. Murphy and L.V. Interrante (eds.) , John Wiley , New York , 1995 ; the preparation of Ba1-xKxBiO3 is described on pp. 198-199 ; J.C. Stark et al. , The Preparation of BaBiO3 and the Superconductor K0.4Ba0.6BiO3 from Mixed Metal Alkoxide Precursors, Mater. Res. Bull. 26 (7) , pp. 623-630 (1991) .
Strontium analogues : S.M. Kazakov et al. , Discovery of a Second Family of Bismuth-Oxide-Based Superconductors, Nature 390 (6656) , pp. 148-150 (1997) ; C. Bougerol-Chaillout et al. , Structural Studies of New Superconducting Bismuthates (Sr,K)BiO3, Physica C : Superconduct. Appl. 341-48 (3) , pp. 1813-1816 (2000) [PDF , 198 KB] .
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 (February , 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 (November , 1988) .
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