Electron-Doped Antifluorites as Superconductors

 

 

The possibility of electron doping antifluorite compounds to obtain metallic solids and maybe superconductors will be examined in this web page . In other related web pages the chemistry of the LaOFeAs family of superconductors was surveyed , and in a sense this present report is a continuation of them . (The references are listed at the end of this web page . Underlined blue hyperlinks can be clicked when online to download the PDF or HTML file , which will open in a new window) . The FeAs layer in LaOFeAs has the anti-litharge structure , in which the iron atoms are tetrahedrally coordinated by the arsenic atoms . A simple Valence Bond analysis of the electronic structure of the iron atoms suggests that the compound's metallic bond electrons are in the iron 4p frontier orbitals . In a tetrahedral ligand environment the p orbitals extend in between the atoms and are thus sterically unhindered by the ligand (arsenic) atoms :

A top view of the litharge/anti-litharge structure in the FeAs layers of LaOFeAs , showing the square net of iron and arsenic atoms .

The p orbitals can overlap continuously between the iron atoms to form sigma and pi XOs (XO = crystal orbital = metallic bond = conduction band) , the electron conduits in the material . Since there are direct ironiron metallic bonds in the LaOFeAs family of compounds , they can formally be classified as iron synthetic metals .

In this essay , prompted by our understanding of the crystal and electronic structures of the FeAs layer in LaOFeAs , we'll study the antifluorites , whose metal atoms also have a tetrahedral coordination by the non-metal atoms . Such compounds might be converted into metallic solids and possibly superconductors by electron doping techniques .

 

Antifluorites

 

The antifluorite crystal structure is the converse of the more familiar fluorite structure , which was named after the common mineral fluorite , calcium fluoride . In fluorite the calcium cations have an eight-fold square prism coordination by the fluoride anions . The two formula fluorides are tetrahedrally coordinated by the calciums :

The fluorite/antifluorite structure . In CaF2 , the red spheres are Ca2+ and the blue spheres are F1- . In the antifluorite compound Li2O , the blue spheres are Li1+ and the red spheres are O2- . This M3D molecular model is based on the sketch of the fluorite structure in the nice solid state chemistry textbook by Smart and Moore . There are no covalent bonds (black lines) in the compounds . The lines were added to illustrate the coordinations clearly , and to outline the cubic symmetry of the crystal . CaF2 and Li2O have ionic bonding only .

The above sketch emphasizes the eight-fold atoms at the expense of the four-fold ones . Here's another view of the fluorite/antifluorite structure , this time with the emphasis on the tetrahedral components :

A top view of the fluorite-antifluorite structure , better illustrating the tetrahedrally coordinated ions .

These "ball-and-stick" models clearly show the ionic coordinations in the lattice . Of course , in reality the ions are all tightly packed together in the solid , but my crystal structure software can't draw them (drawings with closed-packed spheres are shown in textbooks by Wyckoff and Sanderson , for example) . The compact atomic packing in the crystal places the metal M cations in close proximity to one another in three-dimensional planes . As in the anti-litharge structure in the LaOFeAs family , it should be possible to continuously overlap the frontier p orbitals in the M atoms to form sigma and pi XOs as the metallic bonds in properly doped composites .

The mineral fluorite can sometimes be found as large , transparent , octahedral crystals . They are usually coloured by Transition metal impurities (I've seen purple fluorite octahedrons for sale in a mineral shop . Attractive pictures of coloured fluorite samples are presented on this web page ) . A blue variety of fluorite , streaked with white or yellow bands , is still mined in England for ornamental purposes . It's called Blue John , a corruption of the French name "bleu jaune", blue-yellow . Chemically pure calcium fluoride is colourless or white , having no high spin paramagnetic cations which colour solids . Coloured fluorite minerals such as Blue John demonstrate the possibility of doping fluorite , and therefore antifluorite , compounds with Transition metal dopants to obtain novel composites with interesting and possibly valuable optical and electronic properties .

Let's assume the Transition metal M2X antifluorite compounds have 100% ionic bonding .The Crystal Field (CF) and Ligand Field (LF) theories are helpful in analysing their electronic structures in this scenario . The Transition metals are d-block elements , with valence electrons in their d orbitals . In ionic compounds the anions coordinate the metal cations , stabilizing some of the d orbitals and destabilizing (sterically blocking) others , depending on the ligand geometry . Here's a sketch showing the splitting of the d orbitals' energy levels associated with different ligand geometries about the Transition metal cations :

In Transition metal d orbitals the valence electrons have the most energetically stable configurations as unpaired singlets (Hund's Rule) . Also , the orbitals will fill up with electrons from the lowest level upwards (Aufbau principle) . The singlet electrons can be compressed into spin-paired couples by strong CF/LF ligands , such as CO , cyanide , cyclopentadienide anion (cp-) , etc. , but oxide and fluoride anions are very feeble nucleophiles , at the "weak end" of the Spectrochemical Series of CF/LF ligands . They are incapable of compressing the singlet electrons , so their Transition metal compounds are almost always paramagnetic as a result . Transition metal cations with different anions and ligand coordinations have various d orbital electron configurations , as illustrated in the following sketch for several coordinate covalent complexes :

In our hypothetical antifluorite M2X compounds the M metal cations are tetrahedrally coordinated by the X anions , say oxide or fluoride . The tetrahedral CF splitting pattern for the [CoCl4]2- complex shown in the sketch will apply to them . The electronic configuration of the iron atoms in Fe2O would be similar to it (Fe1+ is isoelectronic with Co2+ ; both are 3d7) . Such unpaired singlet electrons in the iron atoms would inhibit any superconductivity in Fe2O , assuming it could be synthesized in a stable antifluorite phase ; and in any case , all of the irons' valence electrons remain in the 3d orbitals , and none are promoted into the empty 4p frontier orbitals . Such a problem with high spin iron doesn't occur in the LaOFeAs compounds , because the iron atoms in them are covalently bonded to the arsenic atoms ; by definition , the iron valence electrons are spin-paired in such materials , permitting superconductivity in them .

From this discouraging analysis the antifluorite compounds seem to be unsuitable for any superconductor applications . However , there might be a rather neat solution to the problem . Instead of using oxide or fluoride anions , the chalcogenide anions could be tried . They are known to be chemical reducers , and such a reducing nature has a profound effect on the electronic structure of any metal cations they are combined with . Sulfide anion is also known to be a very strong nucleophile , almost as much as the cyanide anion , which is a powerful CF/LF ligand .

A nice illustration of this concept is provided by the mineral iron pyrites , which is commonly called "Fools' Gold" ; chemically it is iron(II) disulfide , FeS2 . Pyrites has the cubic rocksalt crystal structure , and this is often easy to observe with the naked eye ; the reader will probably have seen small , shiny , brass-coloured cubes of Fools' Gold a millimeter or two in size :

A fist-sized chunk of iron pyrites from the author's collection .

A study of chemically pure iron(II) disulfide has shown it to be a diamagnetic semiconductor ; its Fe2+ cations (3d6) are in a low spin (spin-paired) condition . The chemically reducing disulfide anions , S22- , octahedrally coordinate and feed electronic charge into them and compress their high spin 3d valence electrons into spin pairs :

Compare this to FeO , also with the rocksalt structure (which is slightly distorted from a cubic symmetry) . FeO is a high spin paramagnetic solid with a pronounced antiferromagnetism at lower temperatures (TN = 198 K , 75 C) :

Oxide anions are electronically inert and are weak CF ligands , and unlike the chalcogen anions have no reducing nature whatsoever :

As can be seen in this tabulation , while oxygen is in the same family of elements as the chalcogens , it certainly isn't like them at all , at least in its redox properties .

Consider the hypothetical compound Fe2Se , which might have the antifluorite structure . The following redox equations show a transfer of electrons from the reducer selenide anions to the iron cations :

2 Fe1+ + 2 e- ---------------> 2 Fe0 ............ E0red = 0.447 V ;

Se2- 2 e- ---------------> Se0 ............ E0ox = 0. 924 V ;

Net :  2 Fe1+  +  Se2-  -------->  (Fe0)2Se0 (c) ............  E0T = 0.477 V .

I don't know what the standard reduction potential of the Fe1+ cation is , so I assumed it to be the same as Fe2+, as shown . Fe(I) is a somewhat obscure chemical species , and is found only in a few coordinate covalent compounds of limited stability . The value for E0T shown above , 0.477 V , would correctly apply to the compound FeSe , which is a low temperature superconductor (Tc = 8 K , first discovered to be so in July , 2008 : F.-C. Hsu et al. , PDF , 736 KB) .

The positive value for E0T indicates that a clean reduction of Fe1+ to Fe0 by selenide should be thermodynamically favourable at STP . The compound Fe2Se would then be a mass of iron(0) and selenium(0) atoms ! However , rather than a reduction of Fe1+ to Fe0 occurring , FeSe coordinate covalent bonds should instead form when the selenide anions donate electron pairs to the iron cations . The iron d3s hybrid orbitals (tetrahedral coordination positions) will receive electron charge from the selenide ligands :

As with the Fe2+ in iron pyrites , the selenide anions should compress the 3d kernel electrons into diamagnetic spin pairs . However , because of the tetrahedral ligand geometry , the d3s orbitals are used for the coordination , making them unavailable for any other kernel electrons . The remaining three kernel electrons will be relocated to the empty and energetically accessible 4p frontier orbitals . The electronic configuration in the iron atoms is now similar to that in fluoride-doped LaOFeAs , whose superconducting Tc = 26 K :

If the selenide anions can succeed in keeping the iron cations in a low spin electronic configuration , Fe2Se could quite possibly become superconducting , although at a rather low temperature .

Selenide is interesting in another context . In some cases in cobalt compounds , for example it can induce antiferromagnetism in the Transition metal atoms to which it is bonded . As discussed in the Iron web page , antiferromagnetism is essential in superconductor parent compounds , so that when they are chemically converted into metallic solids , the antiferromagnetic rgime is also impressed onto the singlet free electrons above EF . That gives them an antiparallel spin orientation relative to each other and helps them to magnetically couple together into Cooper pairs . I believe that the pnictide elements are inducing a mild degree of antiferromagnetism into the iron atoms in the LaOFeAs family of compounds ; that's probably why those materials have been able to support superconductivity at a modest , medium temperature (the current record high Tc is about 56 K for several compounds) . The really strong antiferromagnetism is found in copper(II) oxide and its cuprate derivatives , which is why they still have the highest Tc values to date . Selenide seems to have the "magic touch" when it comes to inducing antiferromagnetism in Transition metal cations , so it should definitely be tried as the X anion in the electron-doped M2X antifluorite compounds .

Moody and Thomas studied the thermodynamics of the M2X , MX , and M2X3 Transition metal compounds (M = Sc to Zn inclusive ; X = O , S , Se , and Te) . They calculated the lattice energies , then enthalpies of formation , DHf , of these compounds , commenting ,

........ the consistent negative values for the DH1a and the DH1b ........... permit the prediction that the chalcogenides M2Ch are unstable . In agreement with this prediction , compounds M2Ch are unknown , except for Cu2Ch where the oxide , sulphide , selenide , and telluride are known . These copper chalcogenides do , however , possess pronounced covalent character (p. 1421) . Note : DH1a applies to the disproportionation reaction , M2Ch -----> 2 M0 + Ch0 ; DH1b applies to the other more likely disproportionation reaction , M2Ch -----> MCh + M0 .

Their calculated results are a reflection of the fact that the Transition metal univalent cations are indeed thermodynamically unstable with respect to disproportionation into the corresponding M0 and M2+ valences , except for copper(I) and silver(I) , whose compounds are mostly stable . Compounds of the earlier Transition metal univalent states are known (if somewhat obscure , as noted above) ; they are reducing agents and must be stabilized against disproportionation by strong pi-donor ligands such as CO , NO , CN- , triphenyl phosphine , and other organic phosphines . Examples are the iron(I) nitrosyl halides , prepared by the reaction :

Fe0 + FeCl2 + 3 NO (g) ----- 70 C -----> Fe(NO)3Cl ;

(dark brown needles , m.p. 110 C subl.)

The dimeric , diamagnetic cobalt(I) compounds [Co(NO)2X]2 , where X = Cl , Br , and I ;

The nickel(I) compounds (Ph3P)4Ni+ X- , where X = Cl , Br , and I . These are yellow/orange solids , paramagnetic (1.92.0 BM) , with tetrahedral Ni(I) .

Selenide and telluride are the strong donor ligands in the proposed low-valent M2X compounds , while the base cations are very stable low redox energy species such as Cr2+ , Mn2+ , Fe2+ , and Co2+ . The "extra" electrons in the compounds making them univalent would be dispersed into empty frontier orbitals and delocalized in a metallic bond throughout the lattice above the base cations . Such metallic bonds might help to stabilize the hypothetical antifluorite compounds . The univalent states in the M atoms might also be stabilized in a compact close-packed system such as the antifluorite crystal structure with strong coordinate covalent bonding from the X anion (such as selenide) into the M cations . This metallic and coordinate covalent bonding , not contemplated in Graham and Moody's study of the M2X compounds , should be sufficient to overcome the energetic destabilization and disproportionation that would otherwise occur in them .

Oxide and fluoride anions contribute only a minor amount of electron density to coordinate covalent bonds , and in any case , they have no access to higher energy level d orbitals , so they can't form a square prism hybrid orbital . Thus , their Transition metal M2X antifluorite compounds would have only ionic bonds , with no additional stabilizing chemical bonds (covalent , coordinate covalent , or metallic) . They would be unstable and disproportionate , as predicted by Graham and Moody .

The Transition metal M2S antifluorites would similarly be unstable . Referring to the Fe2Se Valence Bond sketch above , sulfur can utilize its empty , hypervalent 3d orbitals in certain high energy conditions , such as in fluorinations to form SF4 [sp3dz2 trigonal bipyramid] and SF6 [sp3dz2s octahedron] . However , in the antifluorites sulfide anion doesn't have enough electrons in the square prism hybrid orbital (it has only 8 ; it needs 16) to form the required eight coordinate covalent M–S bonds . On the other hand , selenide can readily use its 3d10 hypervalent orbitals and electrons to form , and fill up with electrons , its square prism d4sp3 hybrid orbital . So the hypothetical Fe2Se antifluorite should be stable , while Fe2S , Fe2F , and Fe2O are predicted – in agreement with Graham and Moody – to be unstable with respect to disproportionation . However , sulfide does have enough valence electron pairs (4 , from 3s2 3p6) to form M–S coordinate covalent bonds using tetrahedral sp3 hybrid orbitals in zinc blende–wurtzite MX compounds , discussed below .

Synthesis of the M2X candidate materials should be straightforward : either by the direct combination of two molar equivalents of the M Transition metal and X chalcogen element , or by the combination of one equivalent of the "normal" MX chalcogenide and an additional equivalent of M element . Since the M cations in these compounds will have an IOS (integral oxidation state , a whole number) of 1+ , and their metallic bond free electrons are expected to be in their frontier p orbitals , we would anticipate the appearance of a spin density wave (SDW) in them as they are cooled down toward Absolute Zero . As suggested in the Doping web page , this SDW may be the localization of the p orbital electrons in sigma MM molecular orbitals (MOs) , which are nodal in nature . That will make the compounds behave like a semiconductor , with a direct electrical conductivitytemperature relationship , and will effectively inhibit the emergence of superconductivity in the lattice .

To prevent the development of the SDW and thereby anticipate superconductivity in the solid , the researcher can chemically design doped composites in which there will be a NIOS (non-integral oxidation state , a fractional number) valence on the M cations . Various doping methods can be used to obtain NIOS composites for testing . With hole doping , electrons are removed from the outer frontier orbitals , reducing the population of free , mobile electrons above EF in the metallic bond XO . Electron doping adds more electrons to the XO . Many experiments (surveyed in the Doping web page) have clearly shown that electron doping is beneficial in the LaOFeAs family of compounds , helping to gradually raise their superconductor transition temperatures to the present (late 2009) Tc ~ 56 K .

One way of doping an IOS M2X antifluorite such as Fe2Se would be to blend with it the corresponding (M+1)2X compound . In the case of Fe2Se , its M+1 element is cobalt :

2 Fe0 + Se0 ------------ (argon) ---------> Fe2Se ;

2 Co0 + Se0 ------------ (argon) ---------> Co2Se ;

x Fe2Se + (1-x) Co2Se -------------------> Fex+2xCo(1-1x)+2-2xSe ,

where x is a mole fraction taken from zero to unity by the researcher .

The doped composite Fex+2xCo(1-1x)+2-2xSe might also be prepared in a one-pot reaction :

2x Fe0 + (2-2x) Co0 + Se0 ------------ (argon) ---------> Fex+2xCo(1-1x)+2-2xSe .

Since the M2X compounds and their composites formally have low-valent M cations , they would be moderately strong reducing agents and so must be carefully protected from atmospheric oxygen and moisture by a blanket of pure , dry argon .

Referring to the VB sketch above of the electronic structure of the hypothetical compound Fe2Se , we see it might be possible to use other Transition metal elements with one less valence electron than iron (Mn) , two less electrons (Cr) , and one more electron than iron (Co) . However , there's a problem with nickel . The nickel(I) in the hypothetical antifluorite Ni2Se is 3d9 electronically , and the nickel cations would almost certainly use the outer tetrahedral sp3 hybrid orbitals for the coordinate covalent bonding , rather than the inner d3s AOs :

This predicted electronic structure for Ni2Se would be similar to that for the molecular nickel(I) compounds (Ph3P)4Ni+ X- [X = Cl , Br , and I] mentioned above , which have tetahedrally coordinated nickel atoms and are paramagnetic .The nickel(I) cations in Ni2Se are similarly predicted to be paramagnetic , with no promotion of any of the 3d valence electrons to the 4p frontier orbitals , which in any case are occupied by the NiSe coordinate covalent bonds . Ni2Se could well be a stable compound with the antifluorite crystal structure ; unfortunately , it would be worthless for formulation into potential superconducting composites , at least in this VB picture .

Six series of M2X chalcogenide composites could be devised , three with selenide anions , and three more with telluride anions : Crx+2xMn(1-1x)+2-2xSe , Mnx+2xFe(1-1x)+2-2xSe , and Fex+2xCo(1-1x)+2-2xSe ; and the corresponding tellurides . The iron-cobalt composites , having the most electrons in their 4p frontier orbitals , should have the highest transition temperatures (if any) of the series , but as mentioned above , they probably would only be around 2030 K or so .

The electronic situation is different in the Group IIB/12 family of elements : zinc , cadmium , and mercury . Their nd valence shells are filled ; for example , Zn0 is 3d10 4s2 electronically . The tetrahedral (sp3) coordination of ligands around Zn2+ cations is strongly almost uniquely favoured ; however , while Cd2+ cations prefer to have an octahedral (sp3dz2s) coordination by ligands , and Hg2+ often has a linear (spx) coordination , they too are known to be tetrahedrally coordinated by anions and other ligands in some of their compounds .

Univalent monoatomic cations are unknown for Zn , Cd , and Hg . The "twin metal" cations (M2)2+ exist for all three elements ; that of mercury is the most stable . The (Hg2)2+ cations in mercury(I) chloride , Hg2Cl2 (commonly called "calomel") , have strong HgHg 6s1 sigma covalent bonds , like a very heavy analogue of the hydrogen molecule's bond . When zinc metal is dissolved in molten anhydrous zinc chloride , a yellow diamagnetic melt containing (Zn2)2+ cations is produced . The ready propensity of the Group 12 elements to form these dimeric metal atom molecules is of concern when considering them in the context of antifluorite M2X compounds intended as metallic solids and as potential superconductors . The following VB sketch shows the possible electronic configuration for Zn1+ cations in the hypothetical compound Zn2Se :

In the antifluorite structure the metal cations are packed closely together in layers in the lattice , so it should be very easy for the 5 s-p frontier orbitals in the zinc cations to overlap continuously in the solid to form a sigma-pi XO ; in particular , the 5s AOs are large , omnidirectional spheres , although they might be partially blocked by the tetrahedral clustering of selenium atoms about the zincs . In the compound Zn2Se , formally with Zn1+ cations (4s1 promoted to 5s1) , there might be a rather strong inclination for the dimeric (Zn2)2+ cations to form throughout the zinc layers . This would , in effect , constitute a very strong SDW localization of metallic bond free electrons in sigma MOs in the material . To prevent the SDW from forming and turning Zn2Se into a semiconductor , some sort of doping of it would be essential . The hypothetical compound Zn2Br , containing mixed-valent Zn0.5+, might be acceptable for this purpose :

2 Zn0 + Se0 ------------ (argon) ---------> Zn2Se ;

1 Zn0 + ZnBr2 ------------ (argon) ---------> Zn2Br ;

x (Zn1+)2Se + (1-x) (Zn0.5+)2Br -------------------> (Zn1/2(x+1)+)2Br1-xSex ,

where x is a mole fraction taken from zero to unity by the researcher . In a one-pot reaction :

(x+3) Zn0 + (1-x) Zn2+Br2 + x Se0 -------------------> (Zn1/2(x+1)+)2Br1-xSex .

The zinc valences could be set by the researcher at the NIOS values of (say) 0.5+ (for Zn2Br) , 0.6 , 0.7 , 0.8 , 0.9 , and the IOS 1+ for Zn2Se : two pure compounds and four intermediate doped composites . It may or may not be possible to to prepare pure Zn2Br in a stable antifluorite phase , if at all ; the one-pot method could circumvent this problem .

The zinc M2X compounds might be difficult or even impossible to prepare if the 4 s-p to 5 s-p energy gap is too great (as illustrated in the textbook by Cotton , Wilkinson , and Gaus) . That is , the empty 5 s-p frontier orbitals may be energetically inaccessible to the Zn1+ cations' 4s1 electrons . Again , referring to the aforementioned energy level illustration , the 5 s-p to 6 s-p energy gap is considerably narrower and more accessible for the 5s1 "extra" electrons in the corresponding Cd1+ M2X compounds . In these latter materials the substrates' and dopants' anions could be matched by size (crystal ionic radius , per Shannon-Prewitt) . Thus , bromide (1.96 ) would be used with selenide (1.98 ) , and iodide (2.20 ) with telluride (2.21 ) . For example ,

(x+3) Cd0 + (1-x) Cd2+Br2 + x Se0 -------------------> (Cd1/2(x+1)+)2Br1-xSex (x = 0 to 1) .

Cd0 m.p. is 321 C , b.p. is 765 C ; CdBr2 m.p. is 567 C ; Se0 m.p. is 217 C , b.p. is 685 C .

Unfortunately in practical terms cadmium is a much more expensive element than the very cheap and abundant zinc ; and it is considered to be a toxic heavy metal and environmental hazard , compared to the relatively innocuous zinc .

Subvalent tin and lead M2X antifluorites could also be studied . A so-called "lead suboxide", Pb2O , is a commercial product used in lead-acid batteries , but careful analyses of it (Bircumshaw and Harris ; see also the earlier report by Denham) showed it to actually be an intimate mixture of Pb0 and PbO . As discussed above , we would expect Pb2O to be unstable with respect to disproportionation as an antifluorite (like Fe2F, Fe2O , and Fe2S) because eight-coordinate oxide anions can't form stabilizing coordinate covalent MO bonds . However , selenide and telluride can use their valence and hypervalent orbitals and electrons to form the required eight MX bonds , per formula unit . It might therefore be possible to synthesize stable M2X antifluorites (M = Sn , Pb ; X = Se , Te) in which the M1+ cations' ns2 np1 valence electrons have been promoted to the next (n+1)s (n+1)p energy levels , mostly in the empty frontier p orbitals , as usual . NIOS electron-doped composites could be prepared using the corresponding M+1 element as the dopant :

2x Sn0 + (2-2x) Sb0 + Se0 ------------ (argon) ---------> Snx+2xSb(1-1x)+2-2xSe ;

2x Pb0 + (2-2x) Bi0 + Se0 ------------ (argon) ---------> Pbx+2xBi(1-1x)+2-2xSe .

Another method of doping an antifluorite like Fe2Se could employ a non-Transition metal divalent cation to replace some of the Fe1+ cations in the lattice . In fact , such a doping technique was first used to obtain mixed-valent compounds having enhanced electrical conductivities . Both oxidizing (hole doping) and reductive (electron doping) procedures to create mixed-valent compounds were investigated by the Dutch solid state physicist E.J.W. Verwey around 1949 , in the Philips Electronics laboratory in Eindhoven , the Netherlands . Examples of two systems studied by Verwey and co-workers are worthy of a brief comment here . The first involved the oxidative doping of nickel(II) oxide with lithium cations . The green insulating NiO was converted into a black semiconductor (its ambient electrical conductivity was ~ 0.8 ohm-1cm-1) . In order to maintain electrical neutrality in the solid , whenever a Ni2+ cation was replaced in the lattice by a Li1+ cation , another Ni2+ cation was oxidized (by oxygen) to Ni3+ :

x  Li2O  +  (1– x)  NiO  +   x O2 (g)  --------- (1200 C , flowing oxygen) -------> Lix Ni1-xO ;

The upper limit of the substitution was 0.9% by mass of  lithium cation . The lithium-doped nickel oxide composites retained the original (slightly distorted) cubic rocksalt structure of the parent NiO . Thus , a mixed-valent Ni(II)–Ni(III) compound was formed , whose electrical conductivity was greatly enhanced compared to the parent NiO insulator .

Another mixed-valent system involving cation doping investigated by Verwey's group was that of the cubic perovskite strontium titanate , whose conductivity rose from 10-7 ohm-1cm-1 to 0.6 ohm-1cm-1 when doped with 1% lanthanum(III) :

x  LaTiO3   +   (1– x)  SrTiO3  ------------ (argon) --------->  Lax Sr1-xTiO3 .

Formally , replacement of a Sr2+ cation by a La3+ cation resulted in the reduction of a Ti(IV) atom to Ti(III) , in order to maintain electrical neutrality in the lattice . The resulting Ti(III)–Ti(IV) mixed-valent composite Lax Sr1-xTiO3 was a semiconductor , compared to the insulating parent compound SrTiO3 with homovalent Ti(IV) .

Verwey's cation substitution method with Lix Ni1-xO might be applied to Fe2Se , but in reverse (that is , a reduction process , rather than oxidation) : replace some of the Fe1+ with (eg.) Mg2+ or Zn2+. To maintain electrical neutrality in the lattice , whenever a Fe1+ cation is replaced by a Mg2+ cation , another Fe1+ must be reduced to Fe0 ; electrons will have been added to the layers of iron atoms . Note that tetrahedrally coordinated magnesium cation has a crystal ionic radius of 0.57 , per Shannon-Prewitt (Zn2+ is 0.60 ) ; that of tetrahedrally-coordinated Fe2+ is 0.63 . I don't know what the radius of Fe1+ (3d7) is , but it's probably comparable to Fe2+ (3d6) . The Mg2+ or Zn2+ should be able to fit comfortably into the Fe2Se matrix without distorting it :

2x Fe0 + (2-2x) Mg0 + Se0 ------------ (argon) ---------> Fey+2xMg2+2-2xSe ;

2x Fe0 + (2-2x) Zn0 + Se0 ------------ (argon) ---------> Fey+2xZn2+2-2xSe ,

where x is a mole ratio taken experimentally between zero and unity , and y indicates a variable NIOS valence state for the iron atoms . In practice , x might not be less than 0.7 or so .

While the scope of the chemistry of the antifluorite M2X compounds for investigation as possible superconductors is much narrower than that of the ZrCuSiAs crystal structure group (which includes the LaOFeAs family of compounds) , they are still of considerable theoretical and practical interest . They lack the unproductive ionic layers of the latter materials , so they have a much higher percentage of electronically active atoms than them . Also , since they are isotropic "three dimensional" compared to the anisotropic layered compounds like LaOFeAs , they will have a three dimensional metallic bond , compared to the planar metallic bonds in the LaOFeAs family . There should thus be a much greater concentration of Cooper pairs and more conduction pathways in the M2X compounds in the superconducting state than in the LaOFeAs family . The former materials should therefore support higher critical currents (Jc) and critical magnetic fields (Hc1) than the latter . Plainly stated , the simple , compact M2X compounds should theoretically give the researcher more "bang for the buck" as superconductors than the LaOFeAs group . This should make them of considerable interest in a practical sense , as potentially valuable technological materials ......... if they can be synthesized as stable metallic antifluorite compounds , and have reasonably high transition temperatures , of course .

 

Zinc Blende (Sphalerite) and Wurtzite Structure Compounds

 

The zinc blende (sphalerite) and wurtzite MX compounds have a diamond-like crystal structure with tetrahedrally coordinated metal M and nonmetal X atoms . The M and X atoms are in alternating layers in these materials :

Above : the cubic zinc blende (sphalerite) structure , in which both the M and X atoms have a tetrahedral coordination . This image was copied from the Wikipedia web page , “Cubic Crystal System”. I thank the author of this sketch , and Wikipedia , for implied permission to reproduce it here on this web page .

Above : the hexagonal wurtzite crystal structure . The M metal cation layers (for example , the smaller silvery spheres) are clearly shown in this sketch , which was copied from the Wikipedia web page , Wurtzite Crystal Structure. Again , my thanks to the author of this sketch , and Wikipedia , for implied permission to reproduce it here on this web page .

Many MX compounds have either the zinc blende or wurtzite crystal structure ; some , like ZnS and AgI , have both . As can be readily seen in the above two sketches , there are planes of close-packed metal M atoms (or cations) in the compounds . The chemical process of electron doping will inject electrons into their empty frontier orbitals , which can overlap continuously in the M layers to form metallic bonds . Such electron-doped materials should thus be metallic solids , and possibly also in a few cases superconductors .

Electron doping of zinc blende and wurtzite compounds might be accomplished by replacing an anion with a lower-valent one ; by replacing the M atom (covalent bonds) by its higher atomic weight M+1 neighbour ; or by replacing the Mn+ cation (ionic bonds) with a suitable M(n+1)+ cation . Lists of zinc blende and wurtzite MX compounds are provided in several textbooks ; I have that one by Ellis et al. at hand , so I'll provide several examples below based on their tabulations .

The chalcogenides of the IIB/12 family (Zn , Cd , and Hg) all have the zinc blende structure ; zinc oxide and the zinc chalcogenides also have the wurtzite crystal form . Since they have no high spin d orbital electrons their divalent cations are all nd10 it might be possible to substitute halide anions for the corresponding chalcogenide anions . As mentioned above , it would probably be prudent to substitute anions of approximately the same size ; selenide (1.98 ) with bromide (1.96 ) , for example . The putative doping agent in this case would be the hypothetical intermediate "ZnBr", which would be formed in situ from the reaction , Zn0 + ZnBr2 -----> "ZnBr". The transient zinc(I) species would , in effect , be adding (Zn2+ + e-) to the lattice :

x Zn0 + x ZnBr2 + (1-x) ZnSe ---------> Zn(2-x)+BrxSe1-x ,

where x is a mole fraction taken experimentally between 0 and 1 (in actual practice , probably not higher than around 0.3 or so) .

While anion electron doping is generally preferable , thereby creating a homoatomic mixed-valent state in the metal M cations (either Robin-Day Class II or IIIB) , in some cases this isn't possible and cation electron doping must be resorted to . For example , as mentioned above , silver iodide has both the zinc blende and wurtzite structures . Since anion doping isn't possible with AgI , cation doping could be tried . The divalent metal cation with the nearest size match to Ag1+ (1.00 , 4-coordinate , per Shannon-Prewitt) is Hg2+ (0.96 , 4-coordinate , i.e. tetrahedral) . The mercury must be doped into the AgI as Hg(I) ["HgI"] , which is Hg2+ + e- . A complicating issue here is that while Hg2I2 is a known compound (according to the CRC Handbook of Chemistry and Physics) , it might not be commercially available (or chemically very stable) , so a reproportionation of Hg0 + HgI2 would be required , as with the Zn0 + ZnBr2 above :

x AgI + (1-x) Hg0 + (1-x) HgI2 ---------> [Agy+]x [Hg2+]1-xI ,

again where x is a mole fraction taken experimentally between 0.5 and 1 , but probably not less than about 0.7 . Such doping should produce a NIOS valence (the exponent "y+") of less than unity in the silver cations , which are normally univalent . That is , electrons will have been added to the silver atoms . Silver(I) is 4d10 5s0 electronically , so the added electrons will likely be located in the silvers' empty 6 s-p frontier orbitals (the 5 sp3 orbitals are used for the AgI coordinate covalent bonds) which can overlap continuously in the lattice to form the metallic bond in the solid . Mercury-doped silver iodide should thus be a metallic material , but it probably wouldn't be superconducting at any temperature , as the host compound AgI is diamagnetic and halide anions aren't known to induce antiferromagnetism in the valence shell electrons of metal cations (the pnictogen elements and selenium probably do induce a mild form of antiferromagnetism in the Transition metal atoms they are bonded to , as noted about halfway up this web page) .

Similarly , copper(I) chloride could be electron-doped with "zinc(I) chloride", which is really Zn2+ + e- + Cl- :

x CuCl (m.p. 430 C) + (1-x) Zn0 (m.p. 420 C) + (1-x) ZnCl2 (m.p. 283 C)

---------> [Cuy+]x [Zn2+]1-xCl ,

with x = 0.5 to 1 and correspondingly y = 0 to 1 . When the dopants Zn0 and ZnCl2 are combined with the substrate CuCl at relatively low doping levels (probably not greater than x = 0.7) the resultant "ZnCl" should assume the structure of the host CuCl lattice , which can be either of the zinc blende or wurtzite variety . Zn2+ has a strong preference for a tetrahedral coordination by anions and ligands , and its crystal ionic radius for a 4-coordinate (tetrahedral) environment is 0.60 per Shannon-Prewitt , which is identical to that of Cu1+. Doping Zn2+ cations into the host Cu1+ lattice will require an equal number of added electrons to maintain electrical neutrality in it . Zinc(II) is a low energy redox species , neither oxidizing nor reducing in nature . However , copper(I) is mildly oxidizing :

Cu1+ + e- ---------> Cu0 ............ E0red = 0.521 V ,

which suggests that the added electrons even though they are derived from the zinc atoms will be located around the copper cations . Copper's 4 s-p orbitals are occupied by the CuCl coordinate covalent bonds , so the extra electrons are expected to enter the empty 5 s-p frontier orbitals (mostly the 5p AOs , since the 5s AOs may be partially blocked by the tetrahedral clustering of chlorides about them , while the 5p AOs are sterically unhindered) . A predominately pi XO should then form in the copper atom layers , which will act as the metallic bond in them .

This zinc-doped CuCl system is particularly interesting , as some controversial research in 1978 suggested that highly compressed (40,000 atm) CuCl briefly became superconducting ; at least , a pronounced "diamagnetic anomaly" accompanied by an upward spike in electrical conductivity around 250 K ( 23 C) was observed . Experimental evidence revealed the presence of copper(0) , i.e. electron-doped Cu(I) , in the compressed CuCl . This Cu(0) might have been produced by some disproportionation of Cu(I) in the lattice :

2 Cu1+ ---------> Cu0 + Cu2+ ............ E0T = 0.368 V .

Possibly zinc-doped CuCl might demonstrate some similarly notable electrical conductivity behaviour , but wouldn't require high pressures to obtain the same sort of result .

Another series of interesting electron-doping experiments might involve the incorporation of M+2 atoms in M metal atom zinc blende or wurtzite lattices . For example , tin sulfide and selenide could be doped into cadmium sulfide and selenide , respectively :

x SnS (m.p. 1153 C) + (1-x) CdS (m.p. ~1000 C subl.) ---------> SnxCd1-xS , and

x SnSe (m.p. 861 C) + (1-x) CdSe (m.p. 1260 C) ---------> SnxCd1-xSe ,

with x = 0 to 1 , but probably not greater than 0.3 or so . Mercury sulfide and selenide could be doped with the corresponding M+2 element , lead , as the respective sulfide and selenide :

x PbS (m.p. 1114 C) + (1-x) HgS (m.p. 584 C subl.) ---------> PbxHg1-xS , and

x PbSe (m.p. 1065 C) + (1-x) HgSe (m.p. 795 C) ---------> PbxHg1-xSe .

The high melting points of the reagents involved might prompt the researcher to investigate a chemie douce route to these chalcogenides , in which the insoluble products are co-precipitated from an aqueous solution at room temperature ; for example ,

x SnCl2 (aq) + (1-x) CdCl2 (aq) --------- [add Na2S . 9H2O (aq) ] ---------> SnxCd1-xS(c) + 2 NaCl (aq)

In the above examples , the substrates (CdS , CdSe , HgS , and HgSe) all have the cubic zinc blende structure , while the dopants (SnS , SnSe , PbS , and PbSe respectively) all have the cubic rocksalt structure . With low dopant levels (x = 0 to about 0.3) the zinc blende structure should be retained by the doped composites . The dopant atoms have two more valence electrons their "inert pairs" : 5s2 for tin(II) , and 6s2 for Pb(II) – than the substrate atoms . When the dopant atoms are incorporated into the zinc blende lattice , hopefully these pairs of electrons will be displaced from their s orbitals (which are then used for the sp3 tetrahedral coordinate covalent bonds) and promoted into the next higher energy level s-p orbitals . The s orbitals will be partially blocked by the clustering chalcogenide atoms , while the p orbitals point in between them , and so aren't sterically hindered by them . So once again , as with the antifluorite and anti-litharge LaOFeAs structures , the mobile , free "extra" electrons might be able to form pi XO metallic bonds in the zinc blende lattices .

Verwey's cation replacement method might also be applied to zinc blendewurtzite compounds . For example , cadmium selenide could be electron-doped with an M+1 element such as aluminum , gallium , or indium (the highly toxic thallium is impractical) :

x CdSe + 1/3 (1-x) Al0 + 1/3 (1-x) Al2Se3 ---------> CdxAl1-xSe ;

x CdSe + (1-x) GaSe ---------> CdxGa1-xSe ; and

x CdSe + 1/3 (1-x) In0 + 1/3 (1-x) In2Se3 ---------> CdxIn1-xSe .

All of the reagents cited (CdSe , Al0 , Al2Se3 , GaSe , In0 , and In2Se3 ) are commercially available .

Several semiconductors used in a wide variety of electronics applications are MX compounds having the diamond-like zinc blende or wurtzite crystal structures . They could be electron-doped (similar to n-doping) by Verwey's cation substitution method . For example , the well-known semiconductor gallium arsenide , GaAs , could be doped by this method :

electron doping with a tetravalent metal atom : x GaAs + (1-x) SnAs ---------> GaxSn1-xAs ;

electron doping with an M+1 non-metal atom : x GaAs + (1-x) GaSe ---------> GaAsxSe1-x ;

x InAs + 1/3 (1-x) In0 + 1/3 (1-x) In2Se3 ---------> InAsxSe1-x .

All of the reagents cited (GaAs , SnAs , GaSe , InAs , In2Se3 , and In0) are commercially available .

Gallium arsenide is a "3+5" semiconductor , having the zinc blende structure . Gallium's three valence electrons are combined with arsenic's five valence electrons to give the electronically very stable octet , per formula unit of GaAs . The eight electrons form the four covalent GaAs tetrahedral bonds . Tin atoms have four valence electrons , so there are 4+5 = 9 covalent bond electrons in SnAs : the octet plus an "extra" electron , which will be located in the gallium's (and/or the tin's) frontier orbitals . In the GaAs–GaSe couple , the dopant GaSe again has nine valence electrons , three from the Ga and six from the Se . The doped composites GaxSn1-xAs , GaAsxSe1-x , and InAsxSe1-x should therefore contain formally mixed-valent (and NIOS) gallium and indium , although the metal atoms are covalently bonded and aren't cations . These doping procedures should inject free electrons into the empty outer frontier orbitals of the gallium and indium , mostly into their p orbitals . Direct Ga–Ga and In–In sigma and pi XO metallic bonds could form in the composites . They should therefore be metallic solids , and possibly even superconductors , although probably only at very low temperatures (in the 0–20 K range) . The electron-doped composites should generally have metallic range electrical conductivities , with an inverse temperature–conductivity relationship (True Metals) .

Indium arsenide might give a better result than gallium arsenide . The indiums' 5 s-p (valence level) to 6 s-p (frontier orbital level) energy transition is narrower and more acccesible to the "extra" electrons than the galliums' 4 s-p to 5 s-p transition .

The corresponding hole-doped composites could also be synthesized by controlled valence techniques , for example :

x GaAs + 1/3 (1-x) As0 + 1/3 (1-x) Zn3As2 ---------> GaxZn1-xAs .

The dopant in this example , "ZnAs", has 2 + 5 = 7 valence electrons in the Zn–As covalent bonds , one less than the usual octet . The doped composites GaxZn1-xAs should therefore have a small percentage of one-electron Zn–As bonds , which will resonate throughout the lattice . The composites should be electrical conductors , but because the metallic bond is in the covalent bond skeleton (which is nodal in nature) , they will be semiconductors (Pseudometals) with a direct temperature-conductivity relationship .

The doped composites could have interesting electronic properties , both separately and when combined in layered structures , as with p- and n-doped silicon and germanium assemblies . Note that in conventional practice , the doping of semiconductor substrates such as silicon with p- and n-type dopants is conducted in the parts per million range , and the resulting doped composites are still semiconductors . However , the doping level of the electron- and hole-doped materials discussed here would be in the stoichiometric range (x = a mole ratio taken experimentally from zero to unity) . Assemblies of such electron- and hole-doped MX compounds might have an enhanced "transistor" behaviour , and as such be valuable new electronic materials . As is well known , semiconductor electronics , because of their low electrical conductivities , i.e. high resistances , have an unfortunate tendency to become quite hot in operation , and require extensive and energy-consuming cooling to protect them from damage and failure (the cooling fans in computers , for example) . Metallic electronics , with much higher conductivities (lower resistances) , should be able to operate at lower temperatures and require less cooling . Electron- and hole-doped MX zinc blende and wurtzite composites could thus provide an invigorating and potentially rewarding new area of research for solid state scientists .

 

References , Notes , and Further Reading

 

related web pages : Prediction of Superconductivity in Transition Metal Chalcogenide Oxides ; Electron Doping of Transition Metal Pnictides and Chalcogenides ; and Exploring Some New Chemistry of Layered Compounds.

crystal orbital : The overlapping of frontier atomic orbitals , physically and energetically above the covalent bond framework in a nonmolecular solid , to form a crystal orbital – metallic bond – in it , has been discussed in another web page , A New Classification of Metallic Solids.

Smart and Moore : L.E. Smart and E.A. Moore , Solid State Chemistry , An Introduction , 3rd edition , CRC/Taylor & Francis , Boca Raton (FL) , 2005 ; Figure 1.39(c) , p. 36 .

software : Molecules 3D Pro from the Molecular Arts Corporation , Anaheim (CA) . Unfortunately , both the company and its software apparently no longer exist .

Wyckoff : R.W.G. Wyckoff , Crystal Structures , 2nd edition , several volumes , Interscience Publishers , New York . Fluorites are discussed in Vol. 1 , pp. 239-243 (1964) . Sketch of the fluorite structure , Fig. IV,1b , p. 240 . Table IV,1 , “Crystals with the Cubic Fluorite Structure”, pp. 241-243 (compounds with an M2X formula are antifluorites) .

Sanderson : R.T. Sanderson , Inorganic Chemistry , Reinhold Publishing , New York , 1967 . A sketch ("close-packed spheres") of the fluorite structure , Fig. 8-7 , p. 117 ; zinc blende structure , Fig. 8-4 , p. 116 ; wurtzite structure , Fig. 8-5 , p. 116 .

Spectrochemical Series : A.F. Holleman , E. Wiberg , and N. Wiberg , Inorganic Chemistry , 1st edition (Engl.) , Academic Press , San Diego (CA) , 2001 ; pp. 1183-1185 . Halide anions are at the "weak end" of the Series . See also F. Basolo and R.C. Johnson , Coordination Chemistry , The Chemistry of Metal Complexes , W.A. Benjamin , New York , 1964 . The Spectrochemical Series of ligands is listed on p. 46 . Crystal field splitting of d orbital energy levels is discussed on pp. 43-49 . Fig. 2-7 , p. 33 , compares high spin [CoF6]3- with low spin [Co(NH3)6]3+. Fig. 2-13 , p. 45 , compares high spin [Fe(H2O)4]3+ with low spin [Fe(CN)6]3-. Sanderson (immediately above) discusses the Spectrochemical Series on pp. 100-101 .

paramagnetic : B.N. Figgis and J. Lewis , “The Magnetic Properties of Transition Metal Complexes”, Prog. Inorg. Chem. 6 , pp. 37-239 , F.A. Cotton (ed.) , Interscience / John Wiley , New York , 1964 ; A. Tressaud and J.M. Dance , Ferrimagnetic Fluorides, Adv. Inorg. Chem. Radiochem. 20 , H.J. Emelus and A.G. Sharpe (eds.) , Academic Press , New York , 1977 ; pp. 133-188 .

strong nucleophile : J. March , Advanced Organic Chemistry , Reactions , Mechanisms , and Structure , 4th edition , John Wiley , New York , 1992 ; Table 10.9 , “Nucleophilicities of Some Common Reagents”, p. 351 .

diamagnetic semiconductor : H. Haraldsen and W. Klemm , “Magnetochemical Investigations . XV . The Magnetic Behaviour of Some Sulfides of Pyrites Structure”, Chem. Abs. 29 , 71365 (1935) ; L. Nel and R. Benoit , “Magnetic Properties of Certain Disulfides”, Chem. Abs. 48 , 3084c (1954) . “FeS2 is practically diamagnetic” ;  A. Wold and K. Dwight , Solid State Chemistry , Synthesis , Structure , and Properties of Selected Oxides and Sulfides , Chapman and Hall , New York , 1993 ; “3d Transition Metal Disulfides with the Pyrite Structure”, pp. 179-182 . “FeS2 is a diamagnetic semiconductor”, p. 180 .

Fe(I) : D. Nicholls , Iron, Ch. 40 , pp. 979-1051 in Comprehensive Inorganic Chemistry , Vol. 3 , A.F. Trotman-Dickenson et al. (eds.) , Pergamon Press , Oxford (UK) , 1973 ; low-valent iron compounds are discussed on pp. 989-1005 .

energetically accessible : F.A. Cotton , G. Wilkinson , and P.L. Gaus , Basic Inorganic Chemistry , 3rd edition , John Wiley , New York , 1995 ; Figure 2-9 (the relative energies of the atomic orbitals as a function of atomic number) , p. 47 . A similar (if somewhat simpler) sketch of the energy levels in atoms is provided by F. Basolo and R.C. Johnson (see above in Spectrochemical Series) , Figure 2-1 , p. 28 . This energy level diagram can be found in many inorganic chemistry textbooks in various forms .

fluoride-doped LaOFeAs : Y. Kamihara , T. Watanabe , M. Hirano , and H. Hosono , Iron-Based Layered Superconductor La[O1-xFx]FeAs (x = 0.05–0.12) with Tc = 26 K”, J. Amer. Chem. Soc. 130 (11) , pp. 3296-3297 (2008) .

antiferromagnetism : V. Johnson and A. Wold , “Crystal Growth and Magnetic Properties of Compositions in the CoS2 : CoSe2 System”, J. Solid State Chem. 2 (2) , pp. 209-217 (1970) : “ …… Se substitution [in CoS2] introduces strong antiferromagnetic interactions between cobalt atoms” (p. 216) ; K. Adachi , K. Sato , and M. Takeda , “Magnetic Properties of Cobalt and Nickel Dichalcogenide Compounds with Pyrite Structure”, J. Phys. Soc. Japan 36 (3) , pp. 631-638 (1969) : CoS2 is ferromagnetic , CoSe2 is antiferromagnetic .

Moody and Thomas : G.J. Moody and J.D.R. Thomas , The Chalcogenides of the First Transition Series, J. Chem. Soc. 1964 , pp. 1417-1422 .

iron(I) nitrosyl halides : see Fe(I) above , p. 1002 .

cobalt(I) compounds : D. Nicholls , Cobalt, Ch. 41 , pp. 1053-1107 in Comprehensive Inorganic Chemistry (see Fe(I) above) ; low-valent cobalt compounds are discussed on pp. 1059-1068 . The dimeric compounds [Co(NO)2X]2 were mentioned on p. 1066 .

nickel(I) compounds : D. Nicholls , Nickel, Ch. 42 , pp. 1109-1161 in Comprehensive Inorganic Chemistry (see Fe(I) above) ; low-valent nickel compounds are discussed on pp. 1115-1124 . The nickel(I) complexes (Ph3P)4Ni+ X- were mentioned on p. 1123 .

Cr2+ : Chromium(II) is a mild reducer [+ 0.407 V to Cr(III)] . The low energy species , in redox terms , is actually chromium(III) .

are known : F.A. Cotton , G. Wilkinson , C.A. Murillo , and M. Bochmann , Advanced Inorganic Chemistry , 6th edition , John Wiley , New York , 1999 ; Table 15-2 , Stereochemistry of Divalent Zinc , Cadmium , and Mercury, p. 600 .

zinc metal is dissolved : D.H. Kerridge and S.A. Tariq , The Solution of Zinc in Fused Zinc Chloride, J. Chem. Soc. A 1967 , pp. 1122-1125 ; see also J.D. Corbett , The Cadmium(I) Ion Cd22+. Raman Spectrum and Relationship to Hg22+ ”, Inorg. Chem. 1 (3) , pp. 700-704 (1962) .

Cotton , Wilkinson , and Gaus : see above for energetically accessible .

Bircumshaw and Harris : L.L. Bircumshaw and I. Harris , Lead Sub-oxide, J. Chem. Soc. 1939 , pp. 1637-1639 ; available (PDF , 275 KB) from the RSC website at : http://www.rsc.org/ejarchive/JR/1939/JR9390001637.pdf .

Denham : H.G. Denham , “Lead Subiodide , and an Improved Method for Preparing Lead Suboxide . The Solubility of Lead Iodide”, J. Chem. Soc. Trans. 111 , pp. 29-41(1917) ; available (PDF , 792 KB) from the RSC website at : http://www.rsc.org/ejarchive/CT/1917/CT9171100029.pdf .

E.J.W. Verwey : Evert Johannes Willem Verweij [Dutch spelling] was born in Amsterdam , Holland , on April 30th , 1905 . Verwey joined the Philips Electronics Research Laboratory in Eindhoven , the Netherlands , in May , 1934 , becoming its Director in June , 1946 . Verwey was the inventor of the controlled valence process in the synthesis of mixed-valent compounds ; he originally referred to it as “induced valence” :

E.J.W. Verwey , “Valence Induite”, Bull. Soc. Chim. France , mises au point D122 (1949) [“Induced Valence”, Chem. Abs. 43 , 6015g (1949)] ; E.J.W. Verwey , P.W. Haayman , and F.C. Romeijn , “Electrical Properties of Metallic Oxides Containing Other Metals as Impurities”, Chem. Abs. 46 , 3424e (1952) ; E.J.W. Verwey et al. , “Controlled-Valency Semiconductors”, Philips Research Reports 5 , pp. 173-187 (1950) ; E.J.W. Verwey , P.W. Haayman , and F.C. Romeijn , “Physical Properties and Cation Arrangement in Oxides with Spinel Structures II. Electronic Conductivity”, J. Chem. Phys. 15 (4) , pp. 181-187 (1947) .

This photo is from the web page “Verweij , Evert Johannes Willem (1905-1981)”, H.A.M. Snelders , Biografisch Woordenboek van Nederland , 2012 . My thanks to the copyright owner .

Controlled valence and mixed-valent compounds are of critical importance to high temperature superconductors and modern electronics . For example , the high Tc cuprate superconductors couldn't be made without controlled valence , as they are Robin-Day Class II mixed-valent compounds . Verwey's pioneering research in the late 1940s and early 1950s with mixed-valent solid state systems laid the foundation for all the great advances in superconductors made almost forty years later .

Verwey recognized that the n- (electron) and p- (hole) doping of semiconductors such as silicon and germanium are essentially controlled valence synthesis techniques . He apparently realized – as I have outlined above – that controlled valence methods could be applied to the zinc blende–wurtzite MX compounds to obtain novel electronic materials :

“cf. Chem. Abs. 44 10422c . A discussion of the semiconducting elements Si and Ge and of nonstoichiometric oxidic semiconductors is followed by a description of the “method of controlled valency” for producing semiconductors ........ Analogous considerations apply to ZnS phosphors in which the fluorescent centers are formed by “activator” ions surrounded by S ions . The activators may be halogen ions or certain trivalent cations [i.e. electron-doping the zinc layers] . This concept points to methods for preparing a new group of phosphors” .

E. J. W. Verwey and F. A. Kroger , “New Views on Oxidic Semiconductors and Zinc Sulfide Phosphors”, Philips Technical Review  13  , pp. 90-95 (1951) .

The above paragraph is a quotation (with some editing) from the corresponding SciFinder Scholar abstract .

Verwey's second great scientific interest was colloid chemistry , to which he contributed dozens of research articles . He retired from the Philips Research Laboratory on December 31st , 1966 , and died (at Utrecht , Holland) on February 13th , 1981 . Unfortunately , his seminal work in mixed-valence chemistry seems to have gone largely unrecognized (internationally , at least) during his lifetime .

My thanks to Ms. Miriam Mobach , Public Relations Department , Philips Electronics Research , Eindhoven , the Netherlands , for Dr. Verwey's biographical information .

nickel(II) oxide : W.J. Moore , Seven Solid States , An Introduction to the Chemistry and Physics of Solids , W.A. Benjamin , New York , 1967 ; Ch. 5 , “Nickel Oxide”, pp. 133-162 .

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

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

silver iodide : AgI has two polymorphs [crystal forms] , a and b . Yellow a-AgI (wurtzite) changes to the orange b-AgI (zinc blende) at 145.8 C . b-AgI is quite unusual in that while the iodine atoms have fixed positions in its crystal lattice , the silver atoms can move about freely . b-AgI is an ionic conductor , with an electrical conductivity at 146 C of 1.3 ohm-1-cm-1 , with the silver cations , not electrons , as the electrical charge carriers . Silver iodide is discussed in some detail by A.F. Wells , Structural Inorganic Chemistry , 3rd edition , Clarendon Press , Oxford (UK) , 1962 ; pp. 174-175 .

chemically very stable : The Merck Index entry for "mercurous iodide" states that pure Hg2I2 is a "bright yellow , amorphous , heavy , odorless powder" that turns green on exposure to light and melts , with some decomposition , at 290 C . A slightly impure green form (containing unreacted Hg0) can be prepared from the direct combination of mercury metal and iodine . Mercurous iodide was apparently once used as an anti-syphilis drug .

CuCl briefly became superconducting : C.W. Chu et al. “Anomalies in Cuprous Chloride”, Phys. Rev. B 18 (5) , pp. 2116-2123 (1978) ; J.A. Wilson , “CuCl : Some Facts and Thoughts on High-Temperature Superconductivity”, Phil. Mag. B 38 (5) , pp. 427-444 (1978) ; G.C. Vezzoli , “CuCl : Electrical and Optical Properties at High Pressure at Temperatures From 400 C to Liquid-Nitrogen Conditions”, Phys. Rev. B 26 (8) , pp. 4140-4145 (1982) .

disproportionation : The positive cell potential (E0T = + 0.368 V) for the disproportionation of Cu(I) to Cu(0) and Cu(II) suggests that the reaction is thermodynamically favourable at STP ; thus , no copper(I) compounds should theoretically exist ! However , redox potentials are based on ionic reactions . If copper(I) chloride was purely ionic in nature , it probably would indeed be unstable and disproportionate . Coordinate covalent bonding is essential to stabilize the Cu(I) valence state . Copper(I) chloride is a white crystalline solid that becomes deep blue at 178 C , melts at 430 C to a dark green liquid , and boils at 1490 C ; it is stable throughout this temperature range (however , Cu1+ is sensitive to air oxidation) . Contrast this with with copper(I) fluoride , whose existence is questionable . It's listed in the CRC Handbook of Chemistry and Physics , and is mentioned in various references [for example , D.A. McCaulay , “Cuprous Fluoride”, U.S. Patent 2817576 (December 24 , 1957)] . However , all the inorganic chemistry textbooks I've checked about it deny its existence ; for example , “Copper(I) fluoride is unknown”, F.A. Cotton , G. Wilkinson , C.A. Murillo , and M. Bochmann , Advanced Inorganic Chemistry , 6th edition , John Wiley , New York , 1999 ; p. 857 . As mentioned above , fluoride anion is a very weak nucleophile , and generally forms rather feeble coordinate covalent bonds . Because there is little if any coordinate covalent bonding in CuF , its ionic-only bonds aren't strong enough to save it from disproportionation into Cu0 and CuF2 , at least at high temperatures ; CuF could conceivably be stable at room temperature , and might be prepared in a pure form in a mild chemical reaction at ambient conditions [McCaulay's synthesis of CuF : Cu0 + HF(aq) ------- (BF3 complex) -------> CuF(c) + H2 (g) ] .

The lesson that CuCl and CuF teaches us is that the hypothetical Transition metal M2X compounds discussed above , with additional stabilization from metallic and coordinate covalent bonds , may actually be isolable in antifluorite phases , despite being “shown mathematically” to be subject to disproportionation .

m.p. 1153 C : E.W. Abel , Tin, Ch. 17 , pp. 43-104 in Comprehensive Inorganic Chemistry , Vol. 3 [see Fe(I) above] ; p. 77 . The m.p. of SnS is given as 882 C in the CRC Handbook of Chemistry and Physics .

m.p. ~1000 C subl. : Cadmium sulfide melts at about 1475 C under 10 atm of argon : B.J. Aylett , “Group IIB”, Ch. 30 , pp. 187-328 in Comprehensive Inorganic Chemistry , Vol. 3 [see Fe(I) above] ; p. 268 . However , it begins to sublime at a much lower temperature under one atmosphere .

m.p. 795 C : N.Z. Boctor and G. Kellerud , “Mercury Selenide Stoichiometry and Phase Relations in the Mercury–Selenium System”, J. Solid State Chem. 62 (2) , pp. 177-183 (1986) .

chemie douce : There are numerous research reports describing the preparation of tin(II) sulfide and selenide by various routes , including chemie douce techniques involving aqueous reaction media at ambient temperature . Three typical such papers include : Y.J. Yang and B.J. Xiang , “A Simple Synthesis of SnS Nanoflakes at Ambient Conditions”, Appl. Phys. A 83 (3) , pp. 461-463 (2006) ; H. Peng , L. Jiang , J. Huang , and G. Li , “Synthesis of Morphologically Controlled Tin Sulfide Nanostructures”, J. Nanoparticle Res. 9 (6) , pp. 1163-1166 (2007) ; P. Pramanik and S. Bhattacharya , “A Chemical Method for the Deposition of Tin(II) Selenide Thin Films”, J. Mater. Sci. Lett. 7 (12) , pp. 1305-1306 (1988) . The cadmium , mercury , and lead components of the composites can be prepared in analogous reactions .

SnS : Tin(II) sulfide actually has a highly distorted rocksalt structure : A.F. Wells (silver iodide above) , p. 527 , and Fig. 50(d) , p. 137 . Distortion in the SnS lattice is undoubtedly caused by the inert pairs of 5s2 electrons on the tin atoms . They are in stereochemically prominent hybrid orbital lobes that cause “bulging” in the structure . In contrast , lead(II) sulfide the mineral galena has a cubic rocksalt lattice . The lead(II) cations are apparently able to disperse their 6s2 inert pair electrons into empty higher energy level orbitals (likely 7 s-p) where they are delocalized to a certain extent , causing galena crystals to have a shiny , metallic appearance and a bluish-gray colour . Such a dispersal of the inert pairs of electrons into the frontier orbitals of the heavier metal atoms in certain compounds is discussed by S.-W. Ng and J.J. Zuckerman , “Where are the Lone-Pair Electrons in Subvalent Fourth Group Compounds ?”, Adv. Inorg. Chem. Radiochem. 29 , pp. 297-325 , H.J. Emelus and H.G. Sharpe (eds.) , Academic Press , Orlando (FL) , 1985 . The success of doped composites such as SnxCd1-xSe and PbxHg1-xSe as metallic solids and possible superconductors critically depends on the ability of the composites' lattices to disperse the dopant Sn 5s2 and Pb 6s2 inert pair electrons into the frontier orbitals of the Cd and Hg host atoms, respectively .

 

 

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