Chromium as the Guest Atom in Super-electride Drude Metals


In the Rocksalts web page the possibility of synthesizing rocksalt Drude electron materials using the Transition metal elements Mn , Fe , Co , and Ni as the guest atoms in MF2 and MCl2 host lattices (M = Mg , Ca , and in the corresponding Transition metal difluoride or dichloride salt) was surveyed . Could chromium similarly be the guest atom in various sorts of Drude electron materials ? In the following presentation the chemistry of chromium in several crystal containers for the super-electrides will be considered in order to answer this question . A crystal container is a common , familiar type of crystal structure that might be modified so that it will contain empty cation vacancies in which the Drude free electron gas will reside . In previous Chemexplore web pages about the Drude electron materials the crystal containers examined were the double wurtzites (Electrons) ; perovskites , ReO3 , and fluorocorundum (Perovskites) ; and as mentioned , the rocksalts . Let's see if Cr0, as Cr2+[**] , can be placed in these particular crystal containers .


Cr2+[**] in Zinc Blendes


Chromium is a base metal , so it's a natural reducer (ie. will spontaneously transfer one or more of its valence electrons to an oxidizer , with a decrease in its system energy) :

Cr0 – 2e -------> Cr2+ ; E0ox = 0.913 V ;

Cr0 – 3e -------> Cr3+ ; E0ox = 0.744 V ;

Cr2+ – e -------> Cr3+ ; E0ox = 0.407 V .

The fairly high standard oxidation potentials (all at STP vs. the SHE) for the Cr0–>Cr2+ and Cr0–>Cr3+ transitions indicate that chromium should indeed be capable , in the redox sense , of forming super-electrides when inserted into suitable host lattices (maybe a little prodding by the solid state chemist might be required) . Note that Cr2+ is itself a moderately strong reducer , while Cr3+ is a low energy species redox-wise , neither reducing nor oxidizing in nature . Chromium in its higher valence states (IV , V , and VI) is strongly oxidizing . All the electrides and super-electrides are reducing agents , some quite strongly so . No oxidizers are permitted in the Electride Clubhouse !

In the Electrons web page the M2+[**]Si2S4 zinc blendes were proposed as possible super-electride Drude electron materials . M = Zn , Cd , and Hg ; [**] is a tetrahedral cation vacancy with two “popped” ns2 valence electrons from the Zn , Cd , and Hg guest atoms in the SiS2 host lattice . Chromium atoms might similarly be inserted into silicon disulfide in a 1 : 2 molar equivalent ratio to synthesize Cr0[ ]Si2S4 , ie. Cr2+[**]Si2S4 , also having the zinc blende crystal structure , in which all the atoms and the cation vacancy [ ] have a tetrahedral coordination to each other :

Synthesis of Cr2+[**]Si2S4 might be accomplished by heating together a thoroughly ground and pelletized reaction mixture of chromium metal powder and silicon disulfide :

Cr0 (m.p. 1875 C) + 2 SiS2 (m.p. 1090 C , sublimes) ------- [grind together , pelletize ,

heat in an inert atmosphere or under a graphite blanket] -------> Cr2+[**]Si2S4 .

As chromium metal is quite refractory , a possible alternative might be to use chromium carbonyl [hexacarbonyl chromium(0)] as the source of the Cr0 guest atoms . Chromium carbonyl is a white , crystalline , air-stable , covalent compound , and is commercially-available at a modest cost . According to the Merck Index (8th edition , 1968 , p. 258) , chromium carbonyl ....... “Sublimes at room temperature ; sinters at 90 C ; decomposes at 130 C ; explodes at 210 C”. Its mixture with SiS2 could be gently heated to expel the CO ligands (toxic , fume hood !) , leaving the chromium atoms loosely bonded to the sulfur atoms in a sort of “premix”. This intermediate would be ground , pelletized , and heated to complete the zinc blende synthesis :

(1) Cr(CO)6 (dec. 130 C) + 2 SiS2 ------- [mix , gently heat] -------> Cr0–Si2S4 + 6 CO (g) ;

(2) Cr0–Si2S4 “premix” ------- [grind , pelletize , heat] -------> Cr2+[**]Si2S4 .

Alternately , chromium(III) sulfide , silicon powder (as the source of the Drude electron gas) , and silicon disulfide might be combined to form the zinc blende :

Cr2S3 (m.p. 1350 C) + Si0 (m.p. 1414 C) + 1 SiS2

------- [grind , pelletize , heat] -------> Cr2+[**]Si2S4 .

A Valence Bond analysis of the chromium zinc blende confirms that all of the Cr , Si , and S valence electrons can be fully accomodated in its crystal structure , with the two Cr0 valence electrons (4s1 and one of the 3d electrons) residing in the empty tetrahedral cation vacancy :

In the silicon disulfide host lattice (GIF image , 55 KB) the sulfur atoms have a bent , two-fold coordination with the tetrahedrally coordinated silicons . The sulfurs have a tetrahedral sp3 configuration with two S–Si covalent bonds and two lone pairs of electrons . In two equivalents of SiS2 there are four sulfur atoms having eight lone pairs of electrons . Four of these lone pairs coordinate the Cr0 atom , popping two of its valence shell electrons into the empty tetrahedral cation vacancy . The remaining four sulfur lone pairs surround the popped chromium electrons in the void space .

The early and middle Transition metal elements extensively use their valence shell d orbitals for forming the hybrid orbitals used in covalent and coordinate covalent bonding to other elements' atoms . The post-Transition metal elements in the p-block of the Periodic Table use their native p orbitals for the hybrids . Thus , the early and middle Transition metals form d3s (tetrahedral) and d5s (octahedral) hybrid orbitals , while the p-block elements form sp3 (tetrahedral) and sp5 (octahedral) hybrids for covalent bonding . In the proposed zinc blendes with tetrahedrally coordinated atoms the d3s hybrid orbital would be used by the Cr(II) for receiving the sulfur lone pairs of electrons , forming the S–>Cr coordinate covalent bonds . The unused Cr(II) 3d4 valence electrons would be compressed into a low spin state by the sulfur atoms pressing down on the Cr2+ kernel .

From this electronic picture of Cr2+[**]Si2S4 we can predict that other M0–(pop)–>M2+[**] reactions might be feasible . The two earlier Transition metal elements , titanium (3d2 4s2) and vanadium (3d3 4s2) might be examined in this context . Note that strongly reducing Ti(II) and V(II) compounds , such as TiCl2 and VCl2 , are well known and are reasonably stable . It might therefore be possible to pop the higher energy 4s2 electrons on Ti0 and V0 to form Ti2+[**]Si2S4 and V2+[**]Si2S4 , respectively . The coordinating sulfur atoms in these zinc blendes should help to stabilize their highly reactive Ti(II) and V(II) kernels :

Ti0 (m.p. 1668 C) + 2 SiS2 ------- [grind , pelletize , heat] -------> Ti2+[**]Si2S4 ;

TiS2 + Si0 (m.p. 1414 C) + SiS2 ------- [grind etc.] -------> Ti2+[**]Si2S4 .

Since Cr , V , and Ti are base metals and reducing agents , it might be possible to use oxygen atoms to coordinate their M(II) kernels instead of the sulfurs . This would be highly advantageous , since silica is an abundant , very cheap , and chemically very stable compound (compared to the somewhat obscure , expensive , and reactive silicon disulfide , which rapidly hydrolyses in humid air to silica and H2S , and apparently smells like rotten eggs as a result) . However , silica is a refractory material (m.p. 1710 C) , and a very high temperature would be required for a reasonably fast zinc blende formation . Syntheses with silica could be carried out in an arc furnace . Several references describing syntheses carried out in an arc furnace : T.B. Reed and E.R. Pollard , “Niobium Monoxide”, Inorg. Synth. 14 , A. Wold and J.K. Ruff (eds.) , McGraw-Hill , New York , 1973 ; pp. 131-134 [PDF , 7530 KB for the entire vol. 14] . This was reprinted in Inorg. Synth. 30 , Nonmolecular Solids , D.W. Murphy and L.V. Interrante (eds.) , John Wiley , New York , 1995 ; pp. 108-110 [PDF , 11,593 KB for the entire vol. 30] . A recommended review of the arc furnace method of syntheses involving refractory materials : T.B. Reed , “Arc Techniques for Materials Research”, Mater. Res. Bull. 2 (3) , pp. 349-367 (1967) [DOI] . Theodore Gray describes a home-made arc furnace in “Melting the Unmeltable”, Popular Science , p. 134 , May , 2004 [JPEG image , 479 KB] .

Cr2O3 (m.p. 2435 C) + Si0 (m.p. 1414 C) + 1 SiO2 (m.p. 1710 C)

------- [grind , pelletize , heat in arc furnace under pure argon] -------> Cr2+[**]Si2O4 ;

V2O3 (m.p. 1970 C) + Si0 + 1 SiO2 ------- [heat in arc furnace] -------> V2+[**]Si2O4 ;

TiO2 (m.p. 1843 C) + Si0 + SiO2 ------- [heat in arc furnace] -------> Ti2+[**]Si2O4 .

Note : The melting points cited in these web pages have been taken mostly from the Alfa-Aesar chemical catalogue (1997) , with a few from the Merck Index (8th edition , 1968) , and several from the CRC Handbook of Chemistry and Physics , 87th edition , CRC Press / Taylor & Francis , Boca Raton (FL) , 2006 . In some cases there was a substantial discrepancy between the values from the three different references . The melting points should therefore be considered as approximate only .

The later Transition metal elements (iron to nickel inclusive) have too many valence electrons for the inner d3s hybrid orbital , and would use the outer sp3 hybrid orbital for the S–>M and O–>M coordinate covalent bonds . The hypothetical iron super-electrides Fe2+[**]Si2S4 and Fe2+[**]Si2O4 are predicted to be strongly paramagnetic in addition to being Drude metals . Their iron atoms (3d6 4s2) would probably have high spin Fe2+ kernels : 3d2x2-y2 3d1z2 – 3d1xy 3d1xz 3d1xz – (4sp3) [**] . Because iron is such a common , abundant , and cheap commodity its super-electride with silica would in particular be quite economically attractive (if successful , of course) :

Fe2O3 (m.p. 1565 C) + Si0 + 1 SiO2 ------- [heat in arc furnace] -------> Fe2+[**]Si2O4 ;

FeS (m.p. 1195 C) + Si0 + 1 SiS2 ------- [heat] -------> Fe2+[**]Si2S4 .


Cr(II) in Distorted Perovskites or Ilmenites


The M2+ Transition metal cations (M = Ti to Ni , inclusive) are too small to act as the A cations in AMX3 perovskites . However , they could be the A cations in the related ilmenite ternary compounds . For example , CaTiO3 is the mineral perovskite , while FeTiO3 is the mineral ilmenite . In perovskites the atoms have a cubic or sometimes distorted cubic packing , while in ilmenites they have a hexagonal packing , as shown in the following sketch :

The above sketch of FeTiO3 was copied from the web page , “Magnetic and Structural Properties of Hemo-Ilmenite Solid Solutions”, from the Metal Physics and Technology Laboratory , ETH Zurich , Switzerland . My thanks to its author and/or copyright owner . Note : Fe2O3 is a corundum , not an ilmenite . A corundum is an ilmenite (AMX3) in which A = M , ie. is M2X3 .

The following simple sketch clearly shows the alternating layers of A and M cations in ilmenites , and the regular occurrence of empty cation voids in every third cation position :

The above graphic was copied from : X.C. Liu , R. Hong , and C. Tian , “Tolerance Factor and the Stability Discussion of ABO3-Type Ilmenite”, J. Mater. Sci. : Mater. Electron. 20 (4) , pp. 323-327 (2009) [DOI] . My thanks to the author and/or copyright owner of this illustration .

In the Perovskites web page the possibility of synthesizing super-electrides having the perovskite crystal structure was discussed . The insertion of one gram-atom equivalent of Na0 , Ca0 , or Cd0 into two gram-formula equivalents of aluminum fluoride was suggested as an experimental procedure to obtain the corresponding super-electrides Na1+[*]Al2F6 , Ca2+[**]Al2F6 , and Cd2+[**]Al2F6 . These hypothetical compounds should have a cubic perovskite crystal structure ; the Na1+, Ca2+ , and Cd2+ cations have just the right sizes to enable such a cubic symmetry .

Similarly , insertion of M0 (1.0 equiv. , M = Ti to Ni inclusive) into AlF3 (2.0 equiv.) should produce the corresponding M2+[**]Al2F6 compounds , assuming the two highest energy valence electrons in the M atoms can be successfully popped into the empty A cation void spaces provided for them :

eg. (1) Cr(CO)6 (dec. 130 C) + 2 AlF3 ------- [mix , heat] -------> Cr0–Al2F6 + 6 CO (g) ;

(2) Cr0–Al2F6 “premix” ------- [grind , pelletize , heat] -------> Cr2+[**]Al2F6 .

In the above example the chromium atoms would first be deposited into the large central void spaces of the aluminum fluoride lattice , which has a distorted ReO3 crystal structure :

The above sketch was copied from the Wikipedia web page Aluminum fluoride . My thanks to the author of this graphic , and Wikipedia , for implied permission to reproduce it here .

Let's assume an intermediate “premix” material , Cr0AlF3 , can form when the chromium atoms nest in the large central cavities of the AlF3 structure . Using the Goldschmidt equation the tolerance factor “t” for this compound can be calculated as t = 1.02 [with crystal ionic radii values of 1.29 for Cr0 (CN = 12) , 0.54 for Al3+ (CN = 6) , and an estimated value of 1.15 for linear fluoride anion (CN = 2)] . Given the approximate nature of the crystal ionic radii – especially that of the linear fluorides – this calculation suggests that Cr0AlF3 might well be a cubic symmetry perovskite .

The “premix” Cr0Al2F6 would be thoroughly ground , pelletized , and annealed at a high temperature under an inert atmosphere of nitrogen or argon , or under a protective blanket of graphite powder , to forcibly pop the chromium atoms' valence electron pairs (4s1 and one of the 3d electrons) into the large , empty A cation void spaces in the lattice . Recalculating the tolerance factor for the component “Cr2+AlF3” in the super-electride compound Cr2+[**]Al2F6 , we find that t = 0.79 , which is much too small for a cubic perovskite . “Cr2+AlF3” could be a distorted perovskite having an orthorhombic or rhombohedral symmetry . According to the Wikipedia Goldschmidt article , AMX3 compounds with a tolerance factor between 0.71 and 0.90 have such a distortion , while those with t < 0.71 are ilmenites . Because the Transition metal M2+ cations are coordinated by fluoride anions , which are weak crystal field ligands , they would almost certainly be in a high spin – and voluminous – condition .

Insertion of the related Transition metal elements into the inert AlF3 host structure in a 1 : 2 molar equivalent ratio should similarly result in the synthesis of either a distorted perovskite or an ilmenite :

eg. Fe(CO)5 (b.p. 103 C) + 2 AlF3 ------- [mix , gently heat] -------> Fe0–Al2F6 + 5 CO (g) ;

(2) Fe0–Al2F6 “premix” ------- [grind , pelletize , heat] -------> Fe2+[**]Al2F6 .


Another Chromium(II) Zinc Blende (or Wurtzite) Super-electride , Cr3[**]Cl4


It might be possible to prepare another type of chromium(II) super-electride in the zinc blende (or wurtzite) crystal container by the insertion of chromium metal into the host lattice of chromium(II) chloride in a 1 : 2 molar equivalent ratio :

Cr0 + 2 CrCl2 -------> Cr3[**]Cl4 .

Chromium(II) chloride has the CaCl2 crystal structure (GIF image , 159 KB) , which is a sort of distorted rutile structure (GIF image , 57 KB ; GIF image , 62 KB) . CrCl2 is a moderately strong reducing agent and is very sensitive to atmospheric oxygen , especially when it's moist or in a water solution . A more practical synthesis of Cr3[**]Cl4 would be to use the oxygen-insensitive , commercially available , and moderately priced chromium(III) chloride as the host lattice , rather than CrCl2 .

Anhydrous chromium(III) chloride has a layered crystal structure , with octahedrally coordinated Cr3+ cations . The following Wikipedia sketch shows a top view of a layer of CrCl3 :

The above sketch was copied from the Wikipedia web page Chromium(III) chloride . Again , my thanks to the author of this graphic , and Wikipedia , for implied permission to reproduce it here .

Anhydrous CrCl3 is described as “violet , lustrous , hexagonal , crystalline scales . Greasy feel” (Merck Index , 8th edition , 1968 , p. 256) . The graphite-like structure of chromium(III) chloride should readily intercalate the Cr0 atoms , which would reduce the Cr(III) to Cr(II) in a thermodynamically favorable reproportionation reaction :

Cr0 – 2e -------> Cr2+ ; E0ox = 0.913 V ;

2 Cr3+ + 2 e -------> 2 Cr2+ ; E0red = – 0.407 V ;

Net reaction : Cr0 + 2 Cr3+ -------> 3 Cr2+ ; E0T = 0.506 V .

The Cr0–CrCl2 route to Cr3[**]Cl4 can be combined with this latter reproportionation reaction so that the more practical CrCl3 can be used in the preparation of the super-electride :

5/3 Cr0 (m.p. 1875 C) + 4/3 CrCl3 (subl. 950 C , m.p. 1152 C) ------> Cr3[**]Cl4 ;

Cr3[**]Cl4 = (Cr2+)3[**] (Cl1-)4 ; or alternately ,

(1) 5/3 Cr(CO)6 (dec. 130 C) + 4/3 CrCl3 ------- [mix , gently heat]

-------> 5/3 Cr0– 4/3 CrCl3 “premix” + 5/3 CO (g) ;

(2) 5/3 Cr0– 4/3 CrCl3 “premix” ------- [grind , pelletize , heat] -------> Cr3[**]Cl4 .

Anhydrous chromium(III) chloride is commercially available , although it's somewhat expensive . The relatively inexpensive Cr(III) chloride hexahydrate , a dark green crystalline salt , can be fully dehydrated by thionyl chloride :

The above graphic was copied from the laboratory manual by G.G. Schlessinger , Inorganic Laboratory Preparations , Chemical Publishing Co. , New York , 1962 ; p. 20 . This useful inorganic chemistry synthesis textbook can be downloaded for free from the library resources web page [DJVU , 4990 KB ; a suitable DjVu reader for your computer can be downloaded for free from . The WinDjView reader v. 1.0.3 for older FAT 32 Windows OS can be downloaded for free from FileHorse] . The dehydration of CrCl3.6H2O using thionyl chloride has also been described by A.R. Pray , “Anhydrous Metal Chlorides”, Inorg. Synth. 5 , T. Moeller et al. (eds.) , McGraw-Hill , New York , 1957 ; pp. 153-156 [PDF , 7324 KB for the entire vol. 5 ; also in vol. 28 , pp. 321-323 , PDF , 18,645 KB for the entire vol. 28] .

The Cr2+ cations in Cr3[**]Cl4 would be tetrahedrally coordinated by chloride anions , which would donate their lone pairs to them , forming stable coordinate covalent Cl–>Cr bonds . The Cr(II) would accept these lone pairs in its tetrahedral d3s hybrid orbital (as with Cr2+[**]Si2S4 , discussed above) :

Note that the 3d x2–y2 and z2 unhybridized native orbitals , with their pairs of unused valence electrons , will protrude into the empty lattice space in between the lobes of the d3s hybrid orbital . No Jahn-Teller distortion is therefore anticipated in the tetrahedral CrCl4 units comprising the three-dimensional crystal structure . Coordinate covalent Cl–>Cr bonding should help to stabilize Cr3[**]Cl4 in the zinc blende (or wurtzite) crystal container :

Cr3[**]Cl4 could prove to be a Drude electron material and a fascinating new electronic compound for future investigation .


Related web pages in this series about Drude electron materials :

A New Picture of Superconductivity : Lightning Bolt Electrons in a Crystal” ;

Perovskites Designed as Drude Metals and Ambient Superconductors” ;

Rocksalts Designed as Super-electrides , Drude Metals , and as Possible High Temperature Superconductors” ;

Lead , Tin , and Bismuth as the Guest Atoms in Super-electride Drude Metals” ;

* Betaines and Electrides : From Sugar Beets and Baby Shampoo to Superconductors ; and ,

* Drude Electron Materials Having Rutile and Layered Structures” .


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