Several Thiospinels Proposed as High Temperature Superconductors

 

This web page is a continuation of the earlier essay , Converting p-Block Element Inert Pairs into Cooper Pairs in 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) . The inert pair enabling mechanism for high temperature superconductivity (HTS) was introduced and discussed in the Inert Pairs web page , and was applied to the design and potential synthesis of mercury(0) and tin(II) HTS candidate compounds . In the following study the 6s2 inert pairs in several thallium(I) , lead(II) , and bismuth(III) compounds will be utilized as Cooper pairs in them in their metallic , superconducting state . Thiospinels have been chosen as the “containers” for the electronically active Tl1+, Pb2+, and Bi3+ cations .

The spinel crystal structure was named after the mineral spinel , which chemically is magnesium aluminate , MgAl2O4 . Most AM2X4 spinels have the “normal” structure , in which the A cation has a tetrahedral coordination by the X anions , while both of the M cations have an octahedral coordination by them ; for example , spinel itself : Mgtet(Aloct)2O4 . A few spinels have the “inverse” structure , in which the A cation has an octahedral coordination by the X anions , while one of the M cations has a tetrahedral coordination by them . The best-known and most widely-studied inverse spinel is undoubtedly magnetite , Fe3O4 , which is Fe3+tet(Fe2+oct Fe3+oct)O4 . The A and M octahedral iron cations were grouped together to emphasize their mixed-valent nature in magnetite . Because the octahedral Fe2+ and Fe3+ cations are in crystallographically identical sites in the lattice , the “extra” 3d6 valence electrons on the iron(II) cations can resonate between the mixed-valent iron atoms . This rapid resonance results in magnetite being a modest electrical conductor (about 100 ohm-1-cm-1 at room temperature) . The tetrahedrally-coordinated Fe3+ cations are incapable of “communicating” with the octahedral iron cations because of an orbital mismatch between the tetrahedral and octahedral crystal fields , and so is electronically inert in magnetite .

The spinel crystal structure was first deduced by the Dutch researchers E.J.W. Verwey and E.L. Heilmann at the Philips Electronics laboratory at Eindhoven , the Netherlands , during World War II , and was published in 1947 . Their sketch of the spinel lattice is reproduced as follows :

One useful way of looking at the spinel structure is to divide a cube into eight “sub-cubes” (formally called octants) , four on the lower deck , and four on the upper deck . An AtetX4 unit and two MoctX3 units are placed in alternating octants . This somewhat complicated arrangement is clearly illustrated in a sketch of the spinel crystal structure from the Chemistry Department of the University of Kiel , Germany (GIF image , 14 KB) .

Most known spinels are oxides , but a few spinel sulfides have been studied . A fascinating example of such a thiospinel is Cu0Ti2S4 , prepared by French researchers back in 1980 , from the insertion of copper metal atoms into the layered compound TiS2 in a 1 : 2 stoichiometric ratio . The tetrahedrally-coordinated copper atoms in this material are zerovalent , and their 5s (probably) orbitals are apparently able to overlap continuously in the lattice to form a crystal-wide sigma orbital (metallic bond) , half-occupied by the copper 4s1 valence electrons . As a result of this electronic structure CuTi2S4 is a metallic solid and is electrically-conducting .

In the three hypothetical thiospinels discussed in this web page the A atoms have been carefully chosen so as to be tetrahedrally coordinated by the sulfur atoms , and are electronically inert . The objective here will be to design normal thiospinels with octahedrally-coordinated , electronically active M atoms (having the inert pairs : M = Tl1+, Pb2+, and Bi3+) . The total cationic charge must be 8+ , to precisely balance the total anionic charge of 8– on the sulfides . Examining the thallium compound first , we must therefore have AVI(Tl1+)2(S2)4 , where A is a Transition metal atom in its hexavalent state . In the well-known chromium(VI) oxide , CrO3 , the chromium atoms are tetrahedrally coordinated by the oxygens in long polymeric chains :

Hopefully Cr(VI) would similarly have a tetrahedral coordination by sulfurs as the A atom in the proposed thiospinels . The target compound would thus be CrTl2S4 , ie. CrVItet(Tl1+oct)2S4 . Given the scarcity of the precursor reagents CrS3 and Tl2S it might be most conveniently synthesized from the direct combination of its three component elements in the correct stoichiometric ratio :

Cr0 (m.p. 1907 C) + 2 Tl0 (m.p. 304 C) + 4 S0 (m.p. 115 C , b.p. 445 C)

------- [HP–HT] ------> CrTl2S4 .

This reaction would have to be carried out under high pressure–high temperature (HP–HT) conditions : first , to contain the volatile sulfur and highly toxic thallium vapours ; and second , to compress the Tl1+ cations together so tightly that their 6s2 inert pairs are promoted up into their respective 7s orbitals . The central concept underlying the inert pair enabling mechanism for high temperature superconductivity is that the inert pairs will become Cooper pairs when they are relocated into the 7s orbitals , which can overlap continuously in the lattice to form a voluminous sigma XO (crystal orbital = polymerized molecular orbital = conduction band = metallic bond) . The free electrons in a metallic bond , while mobile and capable of migrating en mass downfield under an applied potential difference , always remain closely associated with their parent atomic kernels . The pairs of promoted electrons should similarly remain in a close proximity to the underlying Tl , Pb , and Bi kernels , just as they did in their original 6s valence shell orbitals . And as pairs of electrons they will be free and mobile in the lattice , as in ordinary metallic solids . As such they will be Cooper pairs , in the superconducting state , possibly even at ambient temperature !

The redox chemistry of the heavier p-block metal elements , in which the inert pair effect is prominently displayed , was discussed in the Inert Pairs web page . A brief review of it as it applies to the various thiospinels surveyed here might be useful . In the proposed compound CrTl2S4 the chromium(VI) atom would normally be a very powerful oxidizer (for example , in CrO3 and chromic acid) . However , the sulfide anion is a natural reducer and can thus electronically “neutralize” the oxidizing nature of the Cr(VI) in the solid state material :

3 S2- – 6e -------> 3 S0 ; E0ox = 0.476 V ;

2 Cr(VI)* + 6e ----> 2 Cr3+ ; E0red = 1.350 V ;

Net reaction : 2 Cr(VI) + 3 S2- -------> 2 Cr3+ + 3 S0 ; E0T = 1.826 V .

* as acidic chromate anion , (HCrO4), for example in chromic acid .

The large , positive cell potential E0T for the S2- --> Cr(VI) charge transfer indicates that the net redox reaction as written would be thermodynamically spontaneous at STP . Chromium(VI) is a strong enough oxidizer to oxidize thallium(I) cations to thallium(III) :

3 Tl1+ – 6e -------> 3 Tl3+ ; E0ox = – 1.252 V ;

2 Cr(VI)* + 6e ----> 2 Cr3+ ; E0red = 1.350 V ;

Net reaction : 2 Cr(VI) + 3 Tl1+ -------> 2 Cr3+ + 3 Tl3+ ; E0T = 0.098 V .

* as acidic chromate anion , (HCrO4), for example in chromic acid .

However , this latter redox reaction wouldn't occur in the thiospinels because the sulfides have “neutralized” the oxidizing nature of the Cr(VI) in CrTl2S4 .

When the 6s2 inert pairs are promoted into the 7s frontier orbitals the underlying Tl(III) kernels will be very oxidizing and electrophilic (1.252 V to Tl1+, which indicates a strong oxidizer ; in fact the reagent thallium(III) nitrate , “TTN”, is used as an oxidizing agent in organic chemistry) . Once again the sulfide anions coordinating them will transfer some charge to them , thereby neutralizing and cancelling their oxidizing power . The sulfide anions will also tend to electrostatically repel the promoted inert pairs , and to shield them coulombically from the underlying Tl(III) kernels . In other words , the sulfides are essential in these thiospinel compounds to stabilize , in the redox sense , the promoted inert pairs in the 7s orbitals . Without such stabilization , they could drift back to the kernels and spherically surround them again as localized 6s2 inert pairs .

If CrTl2S4 was prepared under “ordinary” conditions we would expect a simple salt-like compound to form : (Tl1+)2(CrS4)2, similar to the oxygen analogue , thallium(I) chromate , (Tl1+)2(CrO4)2, and to the familiar sodium chromate , Na2CrO4 . That is , Tl2CrS4 would consist of a lattice of discrete thiochromate anions , (CrS4)2, interspersed with Tl1+ cations . In contrast CrTl2S4 , as an infinite atomic lattice solid with the normal spinel structure , would have entirely covalent and coordinate covalent bonding , and no ionic bonding at all . The valence electron distribution in CrTl2S4 , per the Valence Bond theory , is shown in the following sketch :

In a normal spinel AM2X4 the A cations have a ligancy of four (tetrahedral) , while the M cations have a ligancy of six (octahedral) . The total cation ligancy is 4 + 6 + 6 = 16 . The total anion (sulfide) ligancy must equal this , so the average ligancy per sulfide is 16 / 4 = 4 , which I have assumed to be tetrahedral (it certainly isn't square planar ; however , it might be a distorted sort of tetrahedral coordination such as is found in the corundum crystal structure , for example) .

The electronic configuration for the chromium atoms in CrTl2S4 is sketched as follows :

Chromium , as a “middle” Transition metal element , prefers to use its 3d valence shell orbitals and electrons for creating the hybrid orbitals (GIF image , 42 KB) used in covalent bonds . It is thus likely to use the d3s tetrahedral hybrid orbital , from a combination of the higher energy 3dxy,xz,yz + 4s native orbitals , as shown in the sketch above . On the other hand , silicon has no access to d orbitals , and as a p block element tends to use its native p orbitals in hybrid orbital formation , hence its use of the familiar sp3 hybrid orbital for tetrahedral bonding . Zinc has just the right number of valence electrons (2) to form a tetrahedral lattice with sulfur (6 ; 6 + 2 = 8 , the very stable octet) , so it doesn't need to use any of its 3d orbitals and electrons , which in any case are very inert chemically . So zinc would also use a sp3 hybrid orbital to form a tetrahedral lattice with sulfur in the thiospinel ZnBi2S4 , exactly as it does in the wurtzite / zinc blende compound ZnS .

The objective in the synthesis of CrTl2S4 as a thiospinel (and not as the ionic Tl2CrS4 !) would be to obtain an octahedral coordination for the Tl(I) cations , squeezing them very tightly , and with the redox assistance of the coordinating sulfide anions , “pop” (promote) the 6s2 inert pairs into the empty 7s frontier orbitals . Thallium , as a p-block element , would be most likely to use its native p orbitals in the formation of an octahedral hybrid orbital for the six Tl–S covalent bonds . The composite sp5 octahedral hybrid orbital can form by the combination of the normal valence shell 6s,p orbitals and two of the hypervalent 7p native orbitals : 6 spz + 6 p2x,y + 7 p2x,y = sp5 .

By a careful “electron counting” we see that after all the valence electrons have settled down around their respective atomic kernels , forming the electronically-stable octets (tetrahedral sulfide anions and chromium) and twelve-sets (octahedral thalliums) , there are two “extra , leftover” valence electrons on each of the thallium atoms . These are – informally , since all electrons are absolutely identical – the 6s2 inert pairs , which have effectively been promoted up into the 7s orbitals . Additionally , the remaining empty 7pz native orbitals can be used as reservoirs for leakage of electron density from the 7s2 orbitals , thus permitting them to function as a metallic bond (the 7s,p conduction band , as in divalent metallurgical metals) . This leakage would occur above Tc , when CrTl2S4 would be in its normal metallic state ; the leakage would cease at and below Tc , when the material would be in its superconducting state and the 7s2 electrons would be the free , mobile Cooper pairs .

Two lighter members of the IIIA/13 family of elements , gallium and indium , also demonstrate an inert pair effect and might similarly be investigated as normal spinel B atoms :

Cr0 (m.p. 1907 C) + 2 Ga0 (m.p. 29.8 C) + 4 S0 (m.p. 115 C , b.p. 445 C)

------- [HP–HT] ------> CrGa2S4 ; and ,

Cr0 (m.p. 1907 C) + 2 In0 (m.p. 156.6 C) + 4 S0 (m.p. 115 C , b.p. 445 C)

------- [HP–HT] ------> CrIn2S4 .

The lower-valent oxides and sulfides of aluminum , Al2O , AlO , and AlS , have been detected , but are metastable only at very high temperatures and cannot be isolated at ambient conditions (ref.) . It therefore seems quite unlikely that a covalent-metallic aluminum thiospinel such as CrAl2S4 , with Al1+, could be synthesized and isolated at STP .

ref. : M. Hoch and H.L. Johnston , “Formation , Stability and Crystal Structure of the Solid Aluminum Suboxides : Al2O and AlO”, J. Amer. Chem. Soc. 76 (9) , pp. 2560-2561 (1954) ; C.N. Cochran , “Aluminum Suboxide Formed in Reaction of Aluminum with Alumina”, J. Amer. Chem. Soc. 77 (8) , pp. 2190-2191 (1955) . See also T. Forland et al. , “Measurements of Phase Equilibria in the Aluminum – Aluminum Sulfide System”, Acta. Chem. Scand. , Series A28 (2) , pp. 226-228 (1974) [PDF , 375 KB ; DJVU , 109 KB] . The compound AlS has a narrow window of stability between 1010 C and its m.p. of 1060 C .

 

SiPb2S4 and SiSn2S4

 

The electronic structure of the hypothetical compound SiPb2S4 , as a normal thiospinel with entirely covalent Si–S and Pb–S bonds , would be identical to that shown above for CrTl2S4 . Silicon can have a tetrahedral coordination when bonded to sulfur , as in the compound silicon disulfide , SiS2 :

This image was copied from the Wikipedia web page , Silicon sulfide . My thanks to the author of this sketch , and Wikipedia , for implied permission to reproduce it here .

The two-dimensional infinite chain structure of silicon disulfide shown above is the “ambient pressure” form of the compound . Under high pressure SiS2 has a more compact three-dimensional lattice , superficially resembling that of silica :

My thanks to the copyright owner of the above graphic and text extract .

This high pressure form of silicon disulfide and its HP–HT synthesis are probably more relevant to the present study of thiospinels than its conventional “ambient pressure” form is . The silicon atoms still have a tetrahedral coordination by the sulfurs in high pressure SiS2 . The sulfur atoms in them have a bent , two-fold coordination by the silicons , as in conventional silicon disulfide . Prewitt and Young comment ,

“A notable point is that the Ge–S–Ge and Si–S–Si angles [109.4] are nearly tetrahedral , thus showing the essential difference between this and the cristobalite [quartz , ie. silica] structures” (p. 536) .

The tetrahedral coordination of the silicon atoms in high pressure SiS2 will undoubtedly be reproduced in the hypothetical SiPb2S4 thiospinel under consideration here .

SiPb2S4 might be synthesized by a combination of the chemical reagents SiS2 and PbS (both commercially available at a modest cost , Alfa-Aesar) under HP–HT conditions :

SiS2 (m.p. 1090 C) + 2 PbS (m.p. 1113 C) ------- [HP–HT] ------> SiPb2S4 .

Lead(II) sulfide is an ionic compound with the cubic rocksalt crystal structure . The Pb(IV) kernels within the Pb2+ cations are spherically surrounded by the localized 6s2 inert pairs . As before with the thalliums in CrTl2S4 these Pb(II) inert pairs must be forcibly “popped” up into their respective 7s orbitals , where they can function as the Cooper pairs in the superconducting state . As with the Tl(III) kernels , the Pb(IV) kernels are powerfully oxidizing (E0red = 1.455 V at STP to Pb2+) . Once more the coordinating sulfide anions would come to the rescue of the promoted inert pairs by transferring some of their charge to the Pb(IV) kernels and so neutralizing their oxidizing nature . The octahedral Pb(IV) kernels and the silicon atoms would be firmly bonded with strong covalent bonds to the sulfide anion ligands , as shown in the sketches above .

SiPb2S4 might also be synthesized by a direct combination of the three component elements :

Si0 (m.p. 1414 C) + 2 Pb0 (m.p. 327 C) + 4 S0 (m.p. 115 C , b.p. 445 C)

------- [HP–HT] ------> SiPb2S4 .

The molten lead and sulfur should combine rapidly to form PbS . Silicon powder will react with sulfur at higher temperatures (500–700 C) to form the SiS2 component , for example as described in European Patent Application 0802161 A1 (K. Yamamoto and N. Ikeda , 1997) , “Method of Manufacturing Silicon Sulfide” (PDF , 248 KB) . The natural abundance and commercial cheapness of these three elements would make this latter synthetic route to SiPb2S4 particularly appealing (if it was successful , of course , and if the compound was a high temperature superconductor) .

Tin might form an analogous thiospinel , SiSn2S4 :

Si0 (m.p. 1414 C) + 2 Sn0 (m.p. 232 C) + 4 S0 (m.p. 115 C , b.p. 445 C)

------- [HP–HT] ------> SiSn2S4 .

Tin(IV) has an octahedral coordination by sulfur in the layered , graphite-like compound tin(IV) sulfide , SnS2 (“Mosaic Gold”) , which has the cadmium iodide crystal structure (GIF image , 24 KB) . As pointed out in the Inert Pairs web page , the 5s,p ---> 6s,p energy gap for the promotion of tin's 5s2 inert pairs up into the 6s frontier orbitals should be readily surmountable , and the underlying Sn(IV) kernels have a relatively low E0red = 0.151 V at STP to Sn2+, ie. they are only weakly oxidizing . These considerations suggest the covalent-metallic thiospinel SiSn2S4 should be readily accessible in an HP–HT synthesis .

 

ZnBi2S4

 

The bismuth(V) kernels in ZnBi2S4 are powerfully oxidizing [E0red = 1.759 V to Bi(III)] , but as in the other thiospinels the chemically reducing sulfides can donate some charge to them and so effectively cancel this oxidizing strength . Very high pressures would be required to convert the ionic precursor Bi2ZnS4 – (Bi3+)2(ZnS4)6– into the covalent-metallic spinel ZnBi2S4 – Zn(II)[Bi(V)]2[(e2)2]2S4 – where [(e2)2]2 are the two Cooper pairs in the superconducting state . It might be prepared by an aqueous route and by solid state methods :

(1) 2 Bi(NO3)3 (aq) + Zn(NO3)2 (aq) -------- [ + 4 (NH4)2S (45% aq , Alfa-Aesar) ] ------->

Bi2ZnS4 (ppt) ------- [HP–HT] ------> ZnBi2S4 ;

(2) ZnS (m.p. 1700 C) + 2 Bi2S3 (m.p. 850 C) ------- [HP–HT] ------> ZnBi2S4 ;

(3) Zn0 (m.p. 420 C) + 2 Bi0 (m.p. 271 C) + 4 S0 (m.p. 115 C , b.p. 445 C)

------- [HP–HT] ------> ZnBi2S4 .

In the last example , the combination of zinc dust and flowers of sulfur is known to be very exothermic , so as a precaution the zinc and bismuth might first be alloyed together and then ground (in an agate mortar) to a coarse powder . Sulfur would presumably be much less reactive with “Bi2Zn” than with finely-divided zinc alone .

The two lighter metalloid members of the VA/15 family , arsenic and antimony , also display an inert pair effect . Their M(III) valence states are low energy , in redox terms , while their M(V) valence states are mildly oxidizing : As(V) + 2e -------> As(III) , E0red = 0.560 V ; Sb(V) + 2e -------> Sb(III) , E0red = 0.671 V . To the best of my knowledge As and Sb have no stable , homovalent , M(IV) compounds . They might form thiospinels analogous to ZnBi2S4 :

(1) ZnS (m.p. 1700 C) + 2 As2S3 (m.p. 312 C , b.p. 707 C)

------- [HP–HT] ------> ZnAs2S4 ;

(2) Zn0 (m.p. 420 C) + 2 As0 (sublimes ~ 615 C) + 4 S0 (m.p. 115 C , b.p. 445 C)

------- [HP–HT] ------> ZnAs2S4 ;

(3) ZnS (m.p. 1700 C) + 2 Sb2S3 (m.p. 550 C) ------- [HP–HT] ------> ZnSb2S4 ;

(4) Zn0 (m.p. 420 C) + 2 Sb0 (m.p. 631 C) + 4 S0 (m.p. 115 C , b.p. 445 C)

------- [HP–HT] ------> ZnSb2S4 .

Both arsenic(V) and antimony(V) form octahedrally coordinated hexahalide complexes such as hexafluoroarsenate and hexafluoroantimonate anions , so they might similarly be able to octahedrally coordinate with the sulfur ligands in the proposed thiospinels .

Phosphorus(V) can be octahedrally coordinated by fluorine , as in the hexafluorophosphate anion , PF6 . Could it be octahedrally coordinated by sulfur atoms , as in the hypothetical covalent–metallic spinel ZnP2S4 – Zntet(PIIIoct )2S4 – and maybe even in the oxygen analogue , ZnP2O4 , ie. Zn(II)[P(V)]2[(e2)2]2O4 ? The exothermic fluorination of phosphorus to produce the colorless , unreactive gas PF5 suggests that with the provision of sufficient energy phosphorus atoms can bridge the substantial 3s,p ---> 4s,p energy gap (GIF image , 37 KB) to create the trigonal bipyramid s2p3 hybrid orbitals [3 sp2x,y + (4s + 3pz) = s2p3] required for the PF5 molecules :

The sp5 octahedral hybrid orbital would be required for PF6 : 3 spz + 3 p2x,y + 4 p2x,y = sp5.

Synthesis of ZnP2S4 might be attempted by the combination of zinc sufide with red phosphorus and flowers of sulfur under very high pressure and temperature conditions :

ZnS + 2P0 (red , m.p. 431 C , sublimes) + 3 S0 (m.p. 115 C , b.p. 445 C)

------- [HP–HT] ------> ZnP2S4 .

ZnO and P2O3 could combine to produce ZnP2O4 . Phosphorus(III) oxide is a known compound (m.p. 173 C) , but is thermally unstable and disproportionates above 210 C . This suggests that P2O3 might be prepared in situ by the reproportionation of P0 and P2O5 : 0.8 P0 + 0.6 P2O5 ------> P2O3 . The overall reaction to form the hypothetical phosphorus spinel would then be :

ZnO + 0.8 P0 (red , m.p. 431 C , sublimes) + 0.6 P2O5 (m.p. 562 C , b.p. 605 C)

------- [HP–HT] ------> ZnP2O4 .

While I don't know of any phosphorus compounds in which the phosphorus atoms are octahedrally coordinated by oxygens , there doesn't seem to be any theoretical impediment to the concept , at least based on the precedent of the hexafluorophosphate anion . If it could be synthesized as a covalent–metallic spinel , ZnP2O4 would be especially interesting as a HTS candidate material . Its phosphorus(V) kernels are low energy , in redox terms ; they wouldn't re-attract the 3s2 inert pairs , now promoted up into the empty 4s frontier orbitals . Thus , ZnP2O4 might be stable at STP as the covalent–metallic spinel Zn(II)[P(V)]2[(e2)2]2O4 .

Note that two Cooper pairs would theoretically be produced per formula weight of ZnBi2S4 and the other thiospinels . In all the other superconductors discovered to date , only one Cooper pair per formula weight of the material has been generated . These “double-barreled” thiospinel superconductors could conceivably possess extraordinary critical current and critical magnetic field physical properties . If they also had very high transition temperatures , they would be “super-duper” superconductors !

 

Oxide Spinel Analogues

 

It would be interesting to examine the oxygen analogues of the thiospinels discussed here for comparison purposes . The oxide precursor reagents are also generally more readily available and are cheaper than their sulfide equivalents , and so are more “practical” than them in laboratory synthesis . Some covalent sulfides , such as silicon disulfide , are also very sensitive to hydrolysis ; SiS2 has a “rotten egg” odour from its ready reaction with atmospheric humidity . The p-block metal oxides , on the other hand , are relatively insensitive to hydrolysis .

The oxide analogue of SiPb2S4 would be SiPb2O4 , the ionic form of which is the well known lead orthosilicate , Pb2SiO4 (m.p. 743 C) . This compound is probably a component of leaded glass , ie. glass with a relatively high PbO content ; such types of glass have high refractive indices . Obviously lead orthosilicate is a non-metallic insulator . Could it be converted into a high temperature superconductor by compressing it at high temperatures in an anvil-type of press , so as to convert it into SiPb2O4 with the normal spinel crystal structure , and with the lead(II) 6s2 inert pairs promoted up into the 7s frontier orbitals ? I suspect that SiPb2O4 might be metastable at these very high temperatures and pressures , but would revert to Pb2SiO4 when it is cooled down to room temperature and the pressure is released .

This is because oxide anion , unlike its chalcogenide cousins in Family VIA/16 , is low energy in redox terms ; that is , it is neither oxidizing nor reducing electrochemically . In that sense it resembles the fluoride anion , which is very difficult to oxidize to elementary fluorine (E0ox = – 2.866 V) , and obviously can't be reduced at all . Oxide anion's E0ox is probably comparable to that of fluoride . Oxide anions are oxidized to oxygen gas in the Hall-Hroult process for the electrowinning of aluminum metal from alumina , dissolved in molten cryolite at ~ 1000 C . This electrochemical process requires huge amounts of electrical energy , which is usually obtained from hydroelectric generation plants located near the aluminum smelters . The chromium trioxide mentioned above is a very powerful oxidizer , and is commonly used in organic syntheses , for example , to oxidize various types of organic molecules by dehydrogenating them . On the other hand , I suspect that the corresponding sulfide , CrS3 , would be relatively unreactive by comparison .

Thus , oxide anions (and oxygen linking atoms in covalent bond systems) are incapable of protecting the “popped” 6s2 inert pairs in the 7s orbitals from the powerfully oxidizing Tl(III) , Pb(IV) , and Bi(V) kernels bonded to the oxygens in CrTl2O4 , SiPb2O4 , and ZnBi2O4 , respectively . These three spinels would undoubtedly be fairly strong chemically oxidizing substances . The promoted metallic bond electrons could infiltrate back through the interatomic voids and rejoin the kernels , thereby spherically surrounding them once again as the localized 6s2 inert pairs .

That's the theory , at least . We do have the interesting example of Cava's famous superconducting perovskite Ba0.6K0.4BiO3 , with an onset transition temperature of Tc = 29.8 K . This material had a crystallographic cubic symmetry , indicating there were no “bulging”, stereochemically prominent inert pairs on the bismuth atoms . The Bi(III) 6s2 inert pairs had been successfully dispersed over the 7s,p conduction band , and the compound was fully stable at STP . However , Ba0.6K0.4BiO3 is a mixed-valent compound (Robin-Day Class II) with respect to its bismuth atoms , which have a NIOS (non-integral oxidation state) valence of ~ Bi(4.4+) . That is , there was a “mixture” of Bi(III) and Bi(V) in the lattice , but these two valence states were perfectly reproportionated into Bi(4.4+) by an ultrafast resonance of the 7s,p metallic bond mobile , free electrons . The proposed oxide spinels CrTl2O4 , SiPb2O4 , and ZnBi2O4 have homovalent Tl(I) , Pb(II) , and Bi(III) respectively , so no such electron resonance between the heavy metal atoms would be possible .

One synthesis approach to these oxide spinels might be via their “ordinary”, non-metallic , oxyanion compounds Tl2CrO4 , Pb2SiO4 , and Bi2ZnO4 ; these purified precursors would then be “cooked” at high pressures and temperatures in an anvil type of press in an attempt (I suspect futile) to prepare their corresponding metallic , superconducting spinels :

(1.1) CrO3 + H2O -------> H2CrO4 -------- [ + Tl2CO2 ] -------> Tl2CrO4

-------- [HP–HT] ------> CrTl2O4 ; alternately by metathesis in a water solution :

(1.2) 2 TlF (aq) + K2CrO4 (aq) -------> Tl2CrO4 (ppt) ------- [HP–HT] ------> CrTl2O4 ;

Tl2CrO4 is a known compound : “yellow crystals”, water solubility 0.03 g/L at 20 C . Thallium(I) fluoride and potassium chromate are very water-soluble : 2450 g/L and 650 g/L at 25 C , respectively . In contrast , TlCl is only slightly soluble in water : 3.3 g/L (20 C) . Thallium(I) sulfate (54.7 g/L) and thallium(I) nitrate (95.5 g/L) might also be used as the Tl1+ source .

(2) 2 Pb(NO3)2 (aq) + Na4SiO4 (aq) -------> Pb2SiO4 ------- [HP–HT] ------> SiPb2O4 ;

(3) 2 SnCl2 (aq) + Na4SiO4 (aq) -------> Sn2SiO4 ------- [HP–HT] ------> SiSn2O4 ;

(4) 2 Bi(NO3)3 (aq) + Zn(NO3)2 (aq) -------- [ + 8 NaOH (aq) ] -------> “Bi2Zn(OH)8

------- [heat , – 4 H2O ] ------> Bi2ZnO4 ------- [HP–HT] ------> ZnBi2O4 .

A second possible technique for preparing the spinels might be to combine the two component oxides directly in the press :

(1) Tl2O (m.p. 579 C , American Elements) + CrO3 (m.p. 197 C , dec. ~ 250 C)

------- [HP–HT] ------> [Tl2CrO4 , not isolated] ------> CrTl2O4 ;

(2) 2 PbO (litharge , m.p. 887 C) + SiO2 (m.p. 1722 C) ------- [HP–HT] ------>

[Pb2SiO4 , not isolated] ------> SiPb2O4 ;

(3.1) 2 SnO (m.p. 1080 C , dec.) + SiO2 (m.p. 1722 C) ------- [HP–HT] ------>

[Sn2SiO4 , not isolated] ------> SiSn2O4 ; alternately ,

(3.2) Sn0 (m.p. 232 C) + SnO2 (m.p. 1630 C) + SiO2 (m.p. 1722 C)

------- [HP–HT] ------> [Sn2SiO4 , not isolated] ------> SiSn2O4 ;

(4) Bi2O3 (m.p. 825 C) + ZnO (m.p. 1974 C) ------- [HP–HT] ------>

[Bi2ZnO4 , not isolated] ------> ZnBi2O4 .

The spinels CrTl2O4 , SiPb2O4 , and ZnBi2O4 would likely be metastable only under the HP–HT experimental conditions , and would revert to their corresponding oxyanion compounds Tl2CrO4 , Pb2SiO4 , and Bi2ZnO4 respectively when cooled and depressurized to STP . The tin spinel SiSn2O4 is the most interesting one of this group . Because its underlying Sn(IV) kernels are weakly oxidizing (E0red = 0.151 V , quite low) , it has a reasonable chance of surviving at STP , and if so it might be a successful high temperature superconductor utilizing the inert pair enabling mechanism .

The essential feature in all these spinel syntheses is their high pressure . Obviously , the oxide and sulfide precursor reagents can all be melted , some at extremely high temperatures , then cooled back to room temperature with no change whatsoever in their chemical and physical properties . Under great pressure , however , the atoms can change their bonding from ionic to covalent (or coordinate covalent) . When this occurs , the inert pairs will be promoted from their 6s to 7s orbitals , with the accompanying formation of a metallic bond throughout the lattice . The sulfides should protect the promoted inert pairs from reabsorption by their parent kernels by neutralizing the strong oxidizing nature of the latter atoms .

Unfortunately , relatively few solid state chemistry research laboratories are equipped with the anvil type of presses that can exert the immense pressures (50,000 atmospheres and higher) , with accompanying high temperatures (~ 800–1000 C) , that will be required for the synthesis of the covalent–metallic thiospinels discussed in this report . Hopefully in the future researchers will become increasingly interested in the high pressure–high temperature syntheses of solid state materials that are accessible only under such extreme experimental conditions .

 

 

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