CuNiO , Ni2OAs , and Related Compounds


In a previous Chemexplore web page I discussed the possibility of creating new metallic compounds by inserting zerovalent atoms of metal elements into unreactive host lattices [underlined blue hyperlinks can be clicked when online to retrieve the article cited . The requested document will open in a new window] . Aluminum fluoride , for example , might serve as the host for a variety of zerovalent noble metal atoms . AlF3 has a distorted rhenium trioxide crystal structure :

This image was copied from the Wikipedia web page , “Aluminium fluoride”. I thank the author of this sketch , and Wikipedia , for implied permission to reproduce it on this web page .

The above model of AlF3 is in the polyhedral style , which I often find somewhat confusing and difficult to understand . I prefer the ball-and-stick representations of crystal structures , but in this case the back-and-forth and up-and-down tilting of the AlF6 polyhedra made a ball-and-stick model very difficult for me to draw with my chemistry software . The small greenish spheres represent fluoride anions , with the aluminum cations hidden at the centers of the octahedrons .

Various types of zerovalent atoms might be inserted into the void spaces in this structure to obtain interesting new perovskite-like compounds . Copper(0) atoms inserted into AlF3 in an equimolar ratio might produce Cu0AlF3 . The tolerance factor t for this hypothetical compound (as a perovskite) was calculated to be t = 0.99 , suggesting it could have a cubic symmetry . CuAlF3 should also be a metallic solid with a good ambient electrical conductivity , as the copper(0) 4s orbitals are quite voluminous and should readily overlap in throughout the crystal lattice to form a sigma XO (crystal orbital = conduction band = metallic bond) which is half-filled with the copper 4s1 valence electrons . It might even be superconducting , but only very close to Absolute Zero , as its metallic bond would resemble that of copper itself and the other metallurgical metals .

In fact , at least one copper(0) compound has been synthesized and studied : copper(0) titanium sulfide . This material , a metallic solid with a good ambient electrical conductivity , was prepared around 1980 by French researchers , by the insertion of copper atoms into the host TiS2 (the references are presented at the end of the text , below) . Titanium disulfide has a layered structure somewhat like graphite , and readily accepted the copper atoms . When heated , the 1Cu : 2TiS2 adduct rearranged into the thiospinel CuTi2S4 .

Copper is a good candidate metal atom for insertion into unreactive host lattices , since it's actually a noble metal in redox terms (although it's generally considered to be a base metal in the industrial and commercial senses) :

Cu1+ + e- -------> Cu0 ; E0red = 0.521 V ;

Cu2+ + 2e- -------> Cu0 ; E0red = 0.3419 V .

Cu2+ + e- -------> Cu1+ ; E0red = 0.153 V .

Both Cu1+ and Cu2+ are mild natural oxidizers ; or conversely , Cu0 is a mild unnatural reducing agent , which is characteristic of noble metals (base metals are natural reducing agents) . Because of this reluctance to transfer any of its valence electrons to a host lattice , copper(0) atoms when inserted into such host structures will usually remain in a zerovalent state . For example , in CuTi2S4 the transfer of an electron from the copper(0) to the titanium(IV) is represented by :

Cu0 – e- -------> Cu1+ ; E0ox = – 0.521 V ;

Ti(IV) + e- -------> Ti(III) ; E0red ~ – 0.055 V .

Net reaction : Cu0 + Ti(IV) -------> Cu1+ + Ti(III) ; E0T = – 0.576 V .

The negative value of the cell potential E0T for the reaction as written indicates that it would not be thermodynamically spontaneous at STP . However , the reverse reaction ,

Cu1+ + Ti(III) -------> Cu0 + Ti(IV) ; E0T = 0.576 V ,

has a positive cell potential and would thus be thermodynamically feasible at STP . In simple terms , the mild oxidizer Cu1+ would spontaneously and irreversibly oxidize the mild reducer Ti(III) when the two species are combined in the same lattice . So , we observe Cu0Ti2S4 and not Cu1+Ti2S4 in actual practice . A similar situation would occur with the copper atoms in Cu0AlF3 .

It might be possible to insert various non-metal zerovalent atoms , such as the pnictogens P , As , Sb ; the chalcogens S , Se , and Te ; and the halogens Cl , Br , and I into AlF3 to obtain novel materials for investigation . However , these are all p-block elements , and their native (unhybridized) p orbitals can't overlap throughout the lattice to produce a nodeless XO , unlike copper and the other noble metals , which have positive wave symmetry s orbitals . The X0–AlF3 adducts would thus only be semiconducting at best .

Other noble or near-noble metals are listed as follows :

Au1+ + e- -------> Au0 ; E0red = 1.692 V ;

Au3+ + 3e- -------> Au0 ; E0red = 1.498 V ;

Au3+ + 2e- -------> Au1+ ; E0red = 1.401 V ;

Ag1+ + e- -------> Ag0 ; E0red = 0.7996 V ;

Hg2+ + 2e- -------> Hg0 ; E0red = 0.851 V ;

Pt2+ + 2e- -------> Pt0 ; E0red = 1.18 V ;

Pd2+ + 2e- -------> Pd0 ; E0red = 0.951 V ;

Rh3+ + 3e- ------->Rh0 ; E0red = 0.758 V ;

Pb2+ + 2e- -------> Pb0 ; E0red = – 0.1262 V ;

Sn2+ + 2e- -------> Sn0 ; E0red = – 0.1375 V .

Surprisingly , even the Group VB/15 metalloids are somewhat noble :

As2O3 + 6 H+ + 6e- -------> 2 As0 + 3 H2O ; E0red = 0.234 V ;

Sb2O3 + 6 H+ + 6e- -------> 2 Sb0 + 3 H2O ; E0red = 0.152 V ;

Bi3+ + 3e- ------->Bi0 ; E0red = 0.308 V .

These elements could all be considered as candidates for insertion into a variety of chemically-inert host lattices , with the objective of preparing novel metallic solids having interesting and technologically useful properties . Such technological applications could include superconductivity , as will be discussed in the following sections .




As with any other successful chemical reaction , a zerovalent metal atom insertion product will be formed if it's sufficiently energetically stabilizing with respect to the starting materials (the element and the host structure) . Chemical bonding and atomic coordinations in the product are the principle determining factors for such an energetic stabilization . The formation of strong covalent or coordinate covalent bonds between the inserted metal atoms and the host lattice atoms is the most desirable outcome of the reaction . The elements across the Periodic Table all have preferential coordinations when forming covalent bonds ; these coordinations are related to the numbers of valence shell electrons and their orbitals involved in the bond formation , and are most simply described , in a qualitative manner , by the Valence Bond theory .

Consider , for example , the insertion of copper(0) into the host lattice of zinc oxide , in an equimolar ratio . The copper(0) atoms will be coordinated by the oxygen atoms , forming strong CuO coordinate covalent bonds . The copper will be bonded as Cu1+ (3d10) by the oxygens , displacing the 4s1 Cu valence electron into a higher energy frontier orbital (LUMO) .

We know from chemical experience that Cu(I) always has a two-fold linear coordination by oxygens in all environments . An example of this observation is the anti-silica crystal structure of copper(I) oxide :

This image was copied from the Wikipedia web page , Copper(I) oxide. Again , my thanks to the author of this sketch , and Wikipedia , for implied permission to reproduce it on this web page .

Each copper(I) atom [pink] has a linear coordination by its oxygen neighbours , while each oxygen [red] has a four-fold tetrahedral coordination by its copper neighbours . This is the converse of the silica structure , in which the silicon atoms are tetrahedrally coordinated by their oxygen neighbours . The oxygens have a two-fold linear coordination to the silicons . Both the Cu2O and SiO2 structures are complicated by an interpenetrating twinning of two offset CuO and SiO chains , respectively .

Returning to the hypothetical compound CuZnO , its possible crystal structure is presented in the following sketch :

The zinc cation (blue sphere) retains its tetrahedral coordination from the zinc oxide host substrate , which has the wurtzite crystal structure (hexagonal , with zinc and oxide both 4 : 4 tetrahedrally coordinated) . The inserted copper(0) atoms (red spheres) form planes of infinite CuO chains with the oxygen atoms (green spheres) . The copper atoms have a linear coordination with them , and are coordinated as Cu1+. The Cu(0) 4s1 valence electrons have undoubtedly been relocated to the energetically accessible 4py,z frontier orbitals . These latter orbitals can overlap continuously along the CuO chains with the neighbouring 2p2y,z oxygen orbitals to form a bilayer metallic bond throughout the crystal lattice .

The oxide anions have a tetrahedral coordination in ZnO , and an octahedral coordination – which is quite satisfactory – by the metal cations in CuZnO .The strong coordinate covalentionic CuO bonds , plus the additional CuO metallic bond , together with the highly favorable Cu(I) linear coordination and an undisturbed zinc(II) tetrahedral coordination , suggest that CuZnO should be an accessible and stable compound . It might be synthesized by the combination of finely divided (200325 mesh) copper powder and zinc oxide under an inert atmosphere . Since ZnO is a refractory solid , a fairly high reaction temperature , and possibly also a applied pressure , might be necessary for a successful synthesis of CuZnO :

Cu0 (m.p. 1085 C) + ZnO (m.p. 1974 C) ------- (heat together , inert atm.) -------> CuZnO

This proposed reaction might also be attempted in an arc furnace , similar to the method used in the synthesis of niobium monoxide by the reproportionation of niobium metal powder and niobium pentoxide .

Derivatives of CuZnO with mixed-valent Cu0Cu1+ might be prepared by doping it with the known compound CuLiO (Migeon and co-workers , 1976) :

Cu2O (m.p. 1235 C) + Li2O (m.p. 1427 C) ------- (heat together , 840 C) -------> CuLiO ;

x Cu0ZnO + (1-x) Cu1+LiO -------> Cu(1-x)+ZnxLi1-xO ,

where x = a mole ratio taken experimentally in steps between 0 and 1 .

Li1+ (crystal ionic radius = 0.59 , 4-coordinate , per Shannon & Prewitt) and Zn2+ (0.60 ) are about the same size , and like Zn2+ , Li1+ is known to prefer a tetrahedral coordination when bonded to oxygens (as in Li2O , which has the antifluorite structure) .

Both CuZnO with homovalent Cu0 and CuZnxLi1-xO with mixed-valent Cu(1-x)+ should be metallic solids with a good ambient electrical conductivity . They could also be superconducting ; I'm guessing that CuZnO might have a Tc in the 15–30 K range . It would definitely not be a high temperature superconductor (HTS) , which is usually understood to be a material with a superconducting transition temperature above 77 K , the boiling point of liquid nitrogen .

There are several important requirements for the occurrence of HTS in a crystalline solid :

* It must be a metallic solid , in particular be a True Metal (GIF image , 41 KB) , which have a nodeless XO metallic bond ; Pseudometals (semiconductors) , with nodal XO metallic bonds , are unsuccessful as superconductors ;

* The material must have some sort of rectilinear crystal structure ; a cubic symmetry is preferable . Metallic solids with a hexagonal symmetry crystal structure , while often excellent electrical conductors , generally aren't superconducting (a number of HCP metal elements such as Cd , La , Os , Re , Ru , Ti , and Tl are superconducting near Absolute Zero) ;

* It must have a bilayer metallic bond , preferably with oxygen atoms providing their 2p electron pairs for the lower layer ; the metal atoms' free electrons then form a very rich population above EF , the Fermi level , and have very short coherence lengths ; and ,

* The metal atoms' free electrons above EF must be organized into an antiparallel spin rgime by an antiferromagnetic (AFM) induction from a nearby metal oxide layer having a strong AFM spin structure . The AFM induction is transmitted into the Pauli paramagnetic metallic bond electrons by electron superexchange through intermediary oxygen atoms (or oxide anions) .

These critical conditions are discussed in detail in several recent Chemexplore web pages , to which the interested reader is referred .

CuZnO and its mixed-valent CuZnxLi1-xO derivatives would be True Metals (have an inverse electrical conductivity–temperature relationship) with a CuO bilayer metallic bond , but they lack the all-important AFM induction from an internal AFM metal oxide source . Their diamagnetic Cu(I) kernels (3d10) would be completely ineffective at any sort of AFM induction . The following sketch illustrates the effect on the superconductor Tc of having AFM induction and a bilayer metallic bond in a wide range of metallic solids :

The hypothetical CuZnO and its derivatives with a metaloxygen bilayer metallic bond but no AFM induction would be placed in Group 4 of the above sketch . Group 4 transition temperatures range from 13.7 K (LiTi2O4) to 29.8 K (Ba0.6K0.4BiO3) , and we could realistically anticipate a Tc for CuZnO in this range .




Insertion of copper(0) into the diamagnetic zinc oxide host lattice would thus be unsuccessful in producing a HTS material . Suppose copper(0) was inserted into an AFM host structure ; that should result in a metallic solid with AFM induction from the host lattice . Nickel oxide has a slightly distorted cubic rocksalt crystal structure at room temperature . It is powerfully AFM , with a very high Nel temperature (TN) of 525 K . Could NiO be used as the host structure for an equimolar quantity of Cu(0) atoms ?

Insertion of copper(0) atoms into NiO can be visualized in several steps . First , here's the NiO rocksalt lattice (blue spheres Ni2+ ; green spheres O2-) :

Next , the NiO layers are separated , layer by layer :

Then , the sheets are restacked so that the oxide anions (green) are aligned along the vertical axis :

Finally , an equimolar quantity of Cu(0) atoms are inserted between the oxide anions , so as to form continuous CuO chains in the structure :

As with CuZnO , these CuO chains are the electrical conduits in the crystal . A proposed electronic structure for the hypothetical compound CuNiO , per the Valence Bond theory , is sketched as follows :

The CuO chains would have a strong coordinate covalentionic bonding between the copper atoms (as Cu1+ cations) and the oxide anions . In effect , the CuNiO structure would consist of a rigid three-dimensional framework of parallel CuO chains with periodic anionic charges , in between which Ni2+ cations are electrostatically coordinated , providing additional ionic bonding in the solid .

As with CuZnO , the copper(0) 4s1 valence electrons would be relocated into the nearby 4py,z frontier orbitals (the 4px orbitals are used in the copper spx linear hybrid orbitals for the coordinate covalent CuO bonds) . They should form an excellent Cu 4py,zO 2py,z bilayer metallic bond surrounding the CuO chains , quite suitable for electrical conduction and superconduction .

CuNiO might be synthesized by the combination of finely divided copper metal powder and pure , stoichiometric nickel(II) oxide at a high temperature , and under an inert atmosphere such as argon . The technique could be similar to the arc furnace method used in the preparation of niobium monoxide , as mentioned above :

Cu0 (m.p. 1085 C) + NiO (m.p. 1957 C) ------- (heat together , inert atm.) -------> CuNiO

On a practical note , most commercial nickel(II) oxide is of the black variety . This NiO is an inexpensive , industrial grade of the chemical , and appears black because it's a mixed-valent compound containing both Ni2+ and Ni3+ . Nickel(III) is a fairly strong oxidizer (1.17 V to Ni2+) and shouldn't be used in this reaction . The interested reader who would like to investigate CuNiO is advised to use only green NiO , which is the stoichiometric grade of the reagent with homovalent nickel(II) . Here's a picture of green nickel(II) oxide from the Internet :

This image was copied from the Wikipedia web page , Nickel(II) oxide. Again , my thanks to the author of this sketch , and Wikipedia , for implied permission to reproduce it on this web page .

If a commercial supplier of pure , green reagent grade nickel(II) oxide can't be located , the researcher could prepare a sample quantity of it from : NiCl (aq) + 2 NaOH (aq) -------> Ni(OH)2 (ppt) + 2 NaCl (aq) ---- (filter ; wash ; dry ; heat under an inert nitrogen or argon atm) -----> NiO + H2O (g) .

An alternate synthesis of CuNiO would use nickel metal powder , thus avoiding the nuisance of trying to obtain pure NiO :

CuO (m.p. 1227 C) + Ni0 (m.p. 1455 C) --------- [HP–HT] -------> CuNiO ,

where HP–HT indicates the use of a high pressure–high temperature apparatus to confine the reaction , which might be mildly exothermic :

Ni0 – 2e- -------> Ni2+ ; E0ox = 0.257 V ;

Cu2+ + 2e- -------> Cu0 ; E0red = 0.3419 V ;

Net reaction : Ni0 + Cu2+ -------> Ni2+ + Cu0 ; E0T = 0.5989 V .

The success of CuNiO as a HTS candidate would critically depend on the coordination of the Ni2+ cations by the oxides . In the CuNiO sketch above the flat sheets of rocksalt NiO have been maintained in the product , resulting in a four-fold square planar coordination of the nickel . Such square net NiO planes would hopefully retain their strong AFM spin rgime , resulting in a powerful AFM induction into the CuO metallic bonds . If this scenario can be realized in experimental practice , CuNiO could be a genuine HTS compound , with a Tc > 100 K .

Square planar coordination of Ni2+ cations by oxide anions is unknown ; Ni2+–O2- coordination in ionic nickelates (as in NiO itself) is always octahedral . Nickel(II) has a square planar bonding only in coordinate covalent complexes with bidentate organic ligands . A well-known example of this sort of complex is nickel(II) bis(dimethylglyoximate) , a dark red , water-insoluble solid which is used in the gravimetric analysis of nickel in various materials (one experiment in my Analytical Chemistry course a long time ago was the nickel DMG gravimetric analysis of a stainless steel alloy) :

If the nickel in CuNiO was octahedral and the copper atoms were linear , the oxides would have to be eight-fold coordinated to them . Eight-fold square prism coordination by oxides (the BCC crystal structure in certain elementary metals) is not unusual . It's found , for example , in antifluorite oxides , an example of which is lithium oxide , Li2O , which has tetrahedral Li1+ and square prism oxides . Studying the sketch of CuNiO above , however , I'm unable to mentally shift the rigid CuO chains , or to reposition the Ni2+ cations , so as to obtain such an eight-fold square prism coordination of the oxides to the copper and nickel atoms .

Even if the square planar coordination of the oxides to the nickels is maintained in CuNiO , what effect would it have on the electronic condition of the Ni2+ cations ? It has been observed experimentally that many square planar coordinate covalent complexes of Ni(II) are diamagnetic , while tetrahedrally and octahedrally coordinated nickel(II) complexes with weak crystal field ligands are paramagnetic, typically with ambient magnetic susceptibilities meff ~ 2.83 BM or so , indicating two unpaired electrons per cation . These observations are readily rationalized by the following Valence Bond sketch :

The weak CF (weakly nucleophilic) ligands cluster about the electrophilic Ni2+ cations , either occupying their outer 4 s-p orbitals with feeble bonding , or sterically blocking them . The Ni(II) 3d8 valence electrons expand to fill up the 3d orbitals . This results in tetrahedral and octahedral Ni(II) complexes being high spin , with two unpaired electrons . The stronger , more nucleophilic CF ligands push down toward the nickel kernel and compress the 3d8 electrons into four spin pairs . These ligands form four NiX coordinate covalent bonds , donating electron pairs into the nickel dsp2 square planar hybrid orbital (per the Valence Bond theory) .

This latter bonding couldn't occur with the square planar NiO in CuNiO because the oxide anions can't form any sort of square planar or octahedral hybrid orbital , which is required on their part for covalent bonding . Oxygen , as a 2 s-p element , can form only sp , sp2 , and sp3 hybrid orbitals , none of which are suitable in this square planar bonding scenario . However , the oxide anions might still be capable of sterically blocking the Ni2+ 3dx2-y2 orbital , whose lobes point toward them along the xy axes . They would be energetically destabilized , at too high an energy level to receive the eighth 3d electron , which would pair up with the seventh electron in the 3dxy orbital :

The Ni2+ would then be diamagnetic , with no AFM induction into the Cu–O metallic bonds .

Another question is : can AFM induction succeed when the metallic bond (in the Cu–O chains) and the AFM induction (from the Ni–O layers) are normal to each other ? In all previous HTS composites (eg. the cuprates with Tc > 100 K) the alternating metallic and AFM layers are in a sandwich-like heterostructure with parallel sheets of atoms . With the Ni2+ cations immediately touching on the metallic bond electrons , the AFM induction in CuNiO could either be extraordinarily effective , or maybe totally ineffective . I'm unable to predict how the nickel 3d8 electrons' spin orientations would interact with the free electrons in the CuO metallic bonds , nor whether it would be HTS or not . The only thing I can guarantee about CuNiO is that it would be a fascinating material to study (if it could be successfully prepared , of course) .


Cu3Cl , CsCu3O , and BaCu3N


I discussed the family of low-valent , mixed-valent copper compounds Cu3Cl , CsCu3O , and BaCu3N in my ebook (pp. 342-347) some time ago , and I thought this might be the appropriate occasion to briefly revisit them in the context of the other related copper compounds above .

Copper(I) chloride is a white , crystalline solid with diamagnetic Cu1+ cations . It becomes deep blue at 178 C , melts at 423 C to a dark green liquid , and boils at 1490 C . It's stable throughout this temperature range ; however , Cu1+ is quite sensitive to air oxidation , especially when in solution . Solid state CuCl has the cubic zinc blende crystal structure , in which both the copper(I) cations and chloride anions are tetrahedrally coordinated (as in the diamond structure) .

Some controversial research in 1978 suggested that highly compressed (40,000 atm) CuCl briefly became superconducting ; a pronounced diamagnetic anomaly accompanied by an upward spike in electrical conductivity around 250 K (–23 C) was reported . Geballe(1) commented on his experience studying compressed copper(I) chloride :

One of the more tantalizing compounds we investigated was CuCl . A visitor of Walt Harrison’s from Russia, Sasha Rusakov , came with some novel ideas about the behavior of the band structure under pressure and the possibility of superconductivity . We investigated this possibility with Paul Chu , who contributed his high-pressure expertise . We observed transient inductive and resistive superconducting-like signals in a narrow range of temperatures around 185 K when the CuCl samples were warmed under pressure . Because these transient signals were reproduced from run to run , I believe they could be due to a metastable superconducting phase or interface resulting from the disproportionation reaction 2 Cu1+ = Cu + Cu2+ . More recently , CuO/Cu interfaces heated and quenched by short high-current pulses have been reported to give indications of superconductivity at very high temperatures” (pp. 12-13) .

Geballe(2) comments further :

“The investigator is most often unable to isolate the suspected superconducting phase or to provide samples to others , and the results have not been reproduced in other laboratories . But there remains a possibility that in some cases the signals are real , and that it comes from an unstable isolated minority phase that may have been produced by an uncontrolled synthesis variable . Possible superconductivity of CuCl under pressure is a good example” (pp. 25-26) .

Experimental evidence revealed the presence of copper(0) in the compressed CuCl . Cu(0) might have been produced by some disproportionation of Cu(I) in the lattice :

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

This positive cell potential indicates that the disproportionation reaction as written is thermodynamically favorable at STP , and that Cu(I) can readily disproportionate to the stabler species , copper(0) and copper(II) . The reverse of this reaction (reproportionation) is used in a student laboratory preparation of CuCl from copper turnings and copper(II) chloride in hydrochloric acid solution . The coordinate covalent Cl–>Cu bonding in CuCl is strong enough to protect the copper(I) cations , under most normal conditions , from disproportionation .

Pure CuCl at STP is nonmetallic and obviously isn't even electrically conducting , let alone superconducting , at any temperature . The strange and still inexplicable (although reproducible) Meissner exclusion effect observed in highly compressed CuCl must therefore be attributed to the presence in the material of the Cu(0) impurities . Copper metal , while a superb electrical conductor , never becomes superconducting , even very close to Absolute Zero . Are the copper atoms being inserted into the host CuCl lattice , or maybe with the by-product CuCl2 , to create an undetected superconducting phase ?

I suggested in the ebook that the hypothetical low-valent , mixed-valent (Robin-Day Class II) copper chloride Cu3Cl might be a superconductor , and if so , could be the impurity in compressed CuCl that was responsible for the unusual diamagnetic anomaly and spike in electrical conduction in it . This compound – tris[copper(0.33+)] chloride – could have the anti-rhenium trioxide crystal structure :

In the above sketch , the green spheres would represent the copper(0.33+) cations , while the red spheres would be the chloride anions .

An analogous compound , copper(I) nitride , Cu3N , is well-known . It's a dark green , crystalline solid that burns in air with an incandescence at 400 C , and decomposes in a vacuum at 450 C . Cu3N has a cubic symmetry anti-ReO3 crystal structure , with a = 3.855 . Nitrogen can't form an octahedral hybrid orbital ; it doesn't have the required six native atomic orbitals , and can form only sp , sp2 , and sp3 hybrid orbitals . The nitride anions' 2s and 2p native orbitals are unhybridized , so there must only be ionic bonding in Cu3N .

Cu3Cl would similarly be ionic , with the addition of a metallic bond . The chloride anions would have an octahedral coordination , as in NaCl . The base of copper(I) cations would have the favorable linear coordination by the chlorides . Expanding the formula of Cu3Cl to (Cu0–Cu1+–Cu0) Cl1- , or (Cu1+–Cu1+–Cu1+) 2e- Cl1- , the two “extra” electrons per formula unit would be located in the bilayer CuCl metallic bond permeating the lattice :

The electronic properties of Cu3Cl (bilayer metallic bond , but no appreciable AFM induction) imply that it would not be a HTS material . It might be superconducting , but its Tc probably wouldn't exceed 30 K or so (I would place it in Group 4 in the “Great Divide” sketch above) .

Cu3Cl might be prepared by the reproportionation of Cu0 and Cu1+ (or Cu2+) into Cu0.33+. Copper(0) atoms would be inserted into the lattice of CuCl , and maybe also into that of CuCl2 :

CuCl (m.p. 423 C) + 2 Cu0 (m.p. 1085 C) --------- [HP–HT] ---------> Cu3Cl

CuCl2 (m.p. 598 C) + 2 Cu0 --------- [HP–HT] ---------> Cu3Cl

It might be helpful to carry out the reaction in a high pressure–high temperature apparatus , possibly using the same experimental conditions Rusakov and co-workers used in their studies of CuCl . A successful synthesis of Cu3Cl , and the demonstration of its superconducting ability , could provide some insight into the mystery of the Meissner exclusion signals from compressed CuCl .

The Meissner exclusion effect can be readily observed in common materials at room temperature . For example , bismuth is notably diamagnetic , and a bulk specimen of the metalloid can repel an imposed magnetic field , as demonstrated in the YouTube video “Bismuth Diamagnetic Properties” [FLV , 5029 KB , run time 1:39] . Another impressive YouTube video , “Diamagnetic Levitation Array” [FLV , 9947 KB , run time 3:06] , shows the diamagnetic levitation of pyrolytic graphite wafers over a block of powerful magnets (probably neodymium-boron-iron) at room temperature : no superconductivity required ! So while a demonstration of the Meissner exclusion and diamagnetic levitation is certainly impressive , the only genuine proof of superconductivity in a material , at any temperature , is the unambiguous demonstration of zero electrical resistivity to a direct current flowing in it .

The bromide and iodide analogues of Cu3Cl could also be examined . Copper(I) fluoride is of questionable stability , and its very existence has been questioned , although it has been mentioned in a couple of references (but not in inorganic chemistry textbooks) . Copper(II) iodide also doesn't exist , as the copper(II) cation oxidizes the iodide anion to elementary iodine .

The oxygen analogue of Cu3Cl is CsCu3O . Its crystal structure would be quite similar to the anti-ReO3 structure shown above , but with the addition of the very large cesium cations in the centers of the supercubes. CsCu3O would resemble the AMX3 perovskites with the same large A cations , but the positions of the smaller M metal cations and the X anions would be reversed in the former material . I've referred to this modified perovskite structure as a semiperovskite” [the term “antiperovskite” is already in use for compounds like Ca3NM (M = P , As , Sb , Bi , Ge , Sn , and Pb , DiSalvo , 1992) and MgCNi3 (Cava , 2001) , where large anions occupy the centers of the supercube cavites] :

CsCu3O : large green sphere : Cs1+ ; red sphere : O2- anion ; blue sphere : Cu0.33+ . The tolerance factor for CsCu3O is calculated to be t = 0.93 , suggesting it could have a cubic symmetry .

Again as with Cu3Cl the chemical bonding in CsCu3O would be ionic–metallic in nature , as outlined in the following sketch :

An excellent Cu–O bilayer metallic bond should be able to form in CsCu3O . It should consequently be a good electrical conductor and maybe a superconductor , the latter only at lower temperatures (probably not > 30 K) . The base of Cu(I) cations is entirely diamagnetic , and there would be little if any AFM induction in CsCu3O , so it's not expected to be a HTS material .

Synthesis of CsCu3O could be carried out in two steps . First , the pure Cu(I) precursor CsCuO could be prepared , as mentioned for CuLiO above (Migeon and co-workers) . Then , two equivalents of copper(0) atoms would be inserted into CsCuO to obtain CsCu3O .

Alternately , CsCu3O might be produced in a single-step procedure :

Cs2CO3 (m.p. 793 C) + Cu2O (m.p. 1800 C , dec) + 2 Cu0 (m.p. 1085 C) ---------- mix together , heat in a stream of pure nitrogen ---------> CsCu3O + CO2 (g) .

The dynamic atmosphere of flowing nitrogen would protect the copper atoms from air oxidation , and flushing of the by-product carbon dioxide from the reactants would help to drive the reaction to completion . All the metallic solids proposed in this web page would be air-sensitive , and would require inert atmospheres of pure , dry nitrogen or argon to protect them from oxidation during their syntheses . Their compressed pellets of reaction mixtures might also be protected from atmospheric oxygen by a layer of graphite powder (GIF image , 20 KB) tamped down lightly over them , in a graphite crucible . A graphite blanket wouldn't be applicable in this last example , where CO2 is expelled as a reaction by-product .

The nitride analogue of Cu3Cl is BaCu3N , and like CsCu3O should have a semiperovskite crystal structure . However , the tolerance factor of BaCu3N is calculated to be t = 0.79 , which suggests that it would be distorted from a cubic symmetry ; possibly the anionic Cu3N layers might be twisted or shifted so as to obtain a stronger electrostatic bond with the barium cations . BaCu3N might best be prepared in a two step sequence :

1. Solid state metathesis :

BaCl2 (m.p. 961 C) + CuCl (m.p. 423 C) + Li3N (m.p. 813 C) -------> BaCuN + 3 LiCl (m.p. 610 C ; it could act as a molten flux) ;

2. Copper atoms insertion :

BaCuN + 2 Cu0 -------> BaCu3N , with Cu0.33+ .

As mentioned near the top of this web page , Cu(I) is a mild oxidizer , and undoubtedly can oxidize nitride anion to a zerovalent nitrogen atom and , of course , molecule . Such an internal redox process probably occurs in heated Cu3N , resulting in its rather low decomposition temperature of 450 C . BaCuN and its derivative BaCu3N probably would be thermally labile as well , and neither should be heated too strongly .


Ni2OAs and Related Compounds


The proposed new material Ni2OAs should be isostructural with the LaOFeAs layered compounds , which are a subset of the broader class of the ZrCuSiAs crystal structure . I've surveyed the LaOFeAs compounds in other Chemexplore web pages , to which the interested reader is referred . A few brief refresher paragraphs on LaOFeAs might be helpful , however .

LaOFeAs is a heterostructure comprised of alternating covalent–metallic FeAs layers (in which the electrical conductivity and superconductivity occur) and electronically inert ionic LaO layers :

Valence-counting shows that an electron has been transferred from the LaO donor layers to the electron-accepting FeAs layers : [La3+O2-]1+ [Fe–As]1- .

The FeAs layers have the anti-litharge crystal structure , comprised of layers of coplanar iron atoms coordinated to a layer of arsenic atoms above and below them . The iron (yellow spheres) and arsenic atoms (black spheres) form a square net in the structure :


In the above sketches , the small silvery spheres represent the inert pairs on the arsenic atoms . I've just noticed that the two colored spheres for the iron and arsenic are reversed in the LaOFeAs sketch above . My apologies to the possibly confused reader !

The iron atoms have a four-fold tetrahedral coordination by the arsenic atoms ; the arsenics have a four-fold square pyramid coordination by the iron atoms (plus a fifth non-bonding position occuped by the inert pair) . A proposed electronic structure of the iron and arsenic atoms in the FeAs layers , per the Valence Bond theory , is presented in the following sketch :

We see in this analysis that the iron atoms receive two extra electrons each , one from the overflowing arsenic atoms , and the other from the LaO donor layer . Thus , they become metallic , even though all of their normal valence shell electrons (3d6 4s2) are completely utilized in the iron kernels and in the Fe–As covalent bonds . The iron 4px,y,z native orbitals are stereochemically unhindered by the arsenic atoms , and they can overlap continuously over the planes of iron atoms to form Fe–Fe metallic bonds (pi XO) , in which the electrical conductivity and superconductivity occur . LaOFeAs can reasonably be described as an iron synthetic metal . Note , however , that the iron 4px,y orbitals can simultaneously overlap in a sigma-type of bonding (+ +) and antibonding (– –) in the x–y plane ; this can result in the formation of ephemeral Fe–Fe covalent bonds , which are a localization of pairs of the extra electrons by a charge density wave (CDW) :

The appearance of a CDW in LaFeAs and its multitude of derivatives can result in them exhibiting semiconductor properties (nodal XO) as they are cooled , and an inhibition of any superconductivity in the material . CDWs are often observed in LaOFeAs compounds having an even number of extra electrons (IOS = integral oxidation state , formally on the iron atoms) ; they can be prevented by carefully doping the parent substrate so that it has an odd number of extra electrons (NIOS = non-integral oxidation state) . This situation was discussed at some length in the Doping web page .

The LaOFeAs superconductors could derive their respectable superconductor transition temperatures from a mild sort of AFM induction from the arsenic atoms . I wondered if the electronically and magnetically inert LaO ionic layer could be replaced by an AFM induction layer . Use of a really strong AFM induction layer , like that from NiO , might result in a significant increase in Tc in the LaOFeAs compounds , possibly into the liquid nitrogen range . Because of the monolayer Fe–Fe metallic bond in them , however , we can't reasonably expect such NiO modified heterostructures to be genuine HTS materials (ie. with Tc > 100 K) .

The layered compound Ni2OAs should result from the replacement of the LaO by NiO , and the FeAs by NiAs , in LaOFeAs . I don't recommend a compound such as NiFeOAs , ie. [Ni2+O2-] [Fe–As] , in which the Ni and Fe atoms may be randomly mixed up , thereby creating a complicated and possibly non-reproducible material . The Transition metal atoms should be identical in both the ionic AFM layers and in the covalent–metallic M–X layers .

NiO is electrically neutral and doesn't donate any electrons to the NiAs layers . If the NiAs layers have the anti-litharge crystal structure , the nickel atoms in them can promote two of their valence electrons (3d8 4s2) to the 4p frontier orbitals , thereby creating a Ni–Ni pi XO metallic bond in the NiAs layers :

The proposed hypothetical compound Ni2OAs would be comprised of alternating NiAs covalent–metallic layers and strongly AFM NiO ionic layers : [Ni2+O2-] [Ni–As] . Nickel arsenide itself has a hexagonal crystal structure :

The large brown spheres represent arsenic atoms (trigonal prism coordination ) ; the smaller blue spheres are nickel atoms (octahedral coordination) .

The general experience is that hexagonal metallic compounds are never superconducting ; only metallic compounds with rectilinear (especially cubic symmetry) structures may/may not be superconducting at lower temperatures . To be used successfully as the metallic and superconducting layer in Ni2OAs the NiAs must assume the anti-litharge crystal structure as sketched above ; then it will have a rectilinear structure – a square net of NiAs atoms – and a satisfactory electronic structure as a superconductor candidate material , being isostructural and isoelectronic with the LaOFeAs compounds . The square net of NiO (from rocksalt nickel oxide) should fit reasonably well on the square net of NiAs , permitting AFM induction to be transmitted from the former to the latter layers .

Ni2OAs might be synthesized by the combination of finely divided nickel metal powder with the combination of 1/3 As0 and 1/3 As2O3 , which is the equivalent of AsO :

2 Ni0 (m.p. 1455 C) + 1/3 As0 (m.p. 615 C , sublimes) + 1/3 As2O3 (m.p. 314 C) ------- [heat , sealed reaction vessel] -------> Ni2OAs , which is [Ni2+O2-] [Ni–As] .

As noted above , As2O3 is a mild oxidizer and can be easily reduced to As0 . The facile reduction of As(III) to arsine gas (AsH3) and subsequently to As0 , is the basis of Marsh's test for arsenic , formerly used in criminal forensics ; see this YouTube video , “Detecting Arsenic – The Marsh Test” [FLV , 7209 KB , run time 2:07] . Also as noted , nickel metal is a mild reducing agent , so it should efficiently reduce the As2O3 to As0 . The Ni2+ cations would sequester the by-product oxide anions in the ionic NiO layers , while the second equivalent of Ni0 would combine with the liberated As0 to form the NiAs covalentmetallic layers .

Both arsenic and arsenic(III) oxide are alarmingly volatile at temperatures considerably below their sublimation and melting points , respectively . Of course , as is well known , they are both intensely toxic substances (Merck Index) . The above reaction should therefore be conducted in a tightly sealed container such as a high pressurehigh temperature apparatus , autoclave , capsule , or ampoule .

The predicted odd number (3) of “extra electrons in the nickel 4p frontier orbitals of Ni2OAs suggests that it probably wouldn't be affected by a CDW , and thus wouldn't require any doping in order to become superconducting . Doping in the LaOFeAs series is usually accomplished by reductive fluorides ; for example , the compound LaO0.89F0.11FeAs has a Tc = 26 K (Kamihara and co-workers , 2008) . In the case of Ni2OAs the reductive doping fluoride would be the hypothetical NiF, Ni1+F1-, which is really Ni2+F1-+ e- , with the e- electron being donated to the NiAs layer . NiF” might be generated in the reaction mixture as an ephemeral species by the combination of Ni0 + NiF2 (anhydrous) :

1 Ni0 + NiF2 + As0 ------- [heat , sealed container] -------> Ni2FAs = [Ni2+F1-]1+ [Ni–As]1-.

The all-fluoride analogue Ni2FAs would also be quite interesting to study . It could be doped into Ni2OAs in a second step to obtain a series of NIOS derivatives of the latter substrate :

x Ni2OAs + (1–x) Ni2FAs ------- [heat , sealed container] -------> Ni2OxF1-xAs ,

where x = a mole ratio between 0 [pure Ni2FAs] and 1 [pure Ni2OAs] , taken experimentally .

A series of Ni2OxF1-xAs composites might be prepared in a single “one-pot” step as follows :

(x+3) Ni0 + (1–x) NiF2 + (1–2x/3) As0 + x/3 As2O3 -------> Ni2OxF1-xAs .

again where x = a mole ratio between 0 and 1 , taken experimentally by the researcher .

The AFM induction in Ni2FAs wouldn't be as strong as in Ni2OAs . Anhydrous nickel(II) fluoride is a yellow , crystalline solid with a tetragonal rutile crystal structure . It sublimes above 1000 C and melts at 1380 C . NiF2 is AFM with a Nel temperature , TN = 83 K , much lower than that of NiO (TN = 525 K) . We would therefore expect a steady decrease of Tc (if any) in the fluoride-doped Ni2OxF1-xAs composites as the mole ratio 1x of fluoride is increased in them .

Many analogues of Ni2OAs , M2OxF1-xX [M = Mn , Fe , Co , and Ni ; X = a pnictogen (P , As , Sb) or chalcogen (S , Se , Te)] , could be designed and synthesized . For example , selenium (4s2 4p4) is the next-door neighbour in the Periodic Table of arsenic (4s2 4p3) , and could conceivably be used in such composites . In an anti-litharge lattice selenium would donate two of its valence electrons to the metal atom M .

The four Transition metal oxides MnO , FeO , CoO , and NiO are all AFM solids with a cubic – slightly distorted at room temperature – rocksalt crystal structure . Their Nel temperatures are 122 , 198 , 291 , and 525 K , respectively . They could act as the AFM induction layer for the anti-litharge MX covalent–metallic layer , where M is the corresponding Transition metal element .

Many binary Mn , Fe , Co , and Ni compounds have the nickel arsenide crystal structure , for example MnAs , MnSb , MnBi , MnTe , FeS , FeSe , FeTe , FeSb , CoS , CoSe , CoTe , CoSb , NiAs , NiSb , NiSe , and NiTe . They might act as the covalent–metallic anti-litharge layer in M2OxF1-xX composites . The NiAs crystal structure seems to be somewhat ambivalent and interchangeable with the anti-litharge form . For example , FeS can have both lattices ; the anti-litharge one is called mackinawite (PDF , 434 KB) , after its naturally-occurring mineral . Wells has noted that FeSe can have both the NiAs and anti-litharge structures . This ambivalence might be advantageous in the synthesis of the new materials from primary chemical reagents . The MO/F cubic ionic layers could serve as a template for the crystallizing MX covalent layers , and thereby impress their rectilinear geometry onto them . The MX layers will then crystallize in the square net anti-litharge structure , rather than in the alternate (and highly undesireable !) hexagonal NiAs form .

Pure FeSe is known to be superconducting at 8 K . Could its transition temperature be raised by layering it with FeO in a heterostructure ?

4/3 Fe0 + 1/3 Fe2O3 + Se0 ------- [HP–HT] -------> Fe2OSe = [Fe2+O2-][Fe–Se] ,

where HP–HT is a high pressure , high temperature apparatus to tightly seal in the toxic , volatile (m.p. 221 C , b.p. 685 C) selenium . Fe0 and Fe2O3 are reproportionated to FeO .

A Valence Bond analysis of Fe2OSe (with anti-litharge FeSe layers) indicates it could be metallic :

However , with two extra electrons in its 4p pi XO , Fe2OSe might be adversely affected by a CDW when it's cooled . Reductive fluoride doping could alleviate this problem :

(1 – x/6) Fe0 + (1–x) FeF2 + x/3 Fe2O3 + Se0 ------- [HP–HT] -------> Fe2OxF1-xSe ,

where x = a mole ratio between 0 [pure Fe2FSe] and 1 [pure Fe2OSe] , taken experimentally .

The anhydrous rutile fluorides MnF2 , FeF2 , CoF2 , and NiF2 are all AFM , although at rather low temperatures : 67 , 79 , 38 , and 83 K , respectively . It might be necessary to strike a balance in the doped Fe2OxF1-xSe composites between the prevention of a CDW in the material (more fluoride) and the AFM induction required to raise Tc (less fluoride) . The transition temperature of the parent FeSe might be raised from 8 K to perhaps around 30 K or so in lightly doped Fe2OxF1-xSe . These materials have a Fe–Fe metallic bond , and so can be considered as iron synthetic metals . But without a bilayer metallic bond , their transition temperatures will be limited to a medium range , even with AFM induction from the FeO layers .

The rather obscure compound nickel monostannide , NiSn , was described as a “silver-white , non-magnetic crystalline powder of sp. gr. 8.44 at 0 C . The calculated sp. gr. is 7.93 . I. Oftedal gave for the dimensions of the hexagonal cells of nickel monostannide , NiSn , c = 5.174 , a = 4.081 , or a:c = 1.268” (PDF , 39 KB) . I discussed the intermetallic compound iron monostannnide , FeSn , in some detail in the Valence Bond web page . The crystal structure of NiSn probably is quite similar to that of FeSn :

NiSn would be very interesting to study as a superconductivity precursor for two reasons . First , it should be an excellent electrical conductor , comparable to that of FeSn . The ambient electrical conductivity of FeSn was found to be about 12,500 ohm-1cm-1 (parallel) , and ~ 15,400 ohm-1cm-1 (perpendicular) . Stenstrm reported a “residual resistivity” [in conductivity terms] of FeSn cooled in liquid helium at 4.2 K of 833,000 ohm-1cm-1 (parallel) and 2.9 million ohm-1cm-1 (perpendicular) . The higher the electrical conductivity , the more free electrons in the metallic bond there are , so the greater the density of the free electrons in the lattice . Their average coherence lengths will therefore be shorter , with a higher transition temperature if superconductivity can appear in the material .

Second , the corresponding AFM induction layer to NiSn would be NiO , which has the highest Nel temperature (TN = 525 K) of the four rocksalt AFM oxides considered in this study . I commented elsewhere : “The rule of thumb here is : maximum electrical conductivity + maximum AFM induction = maximum superconductor Tc. The Ni2OxF1-xSn composites , with their highly metallic NiSn and strongly AFM NiO layers , would be excellent superconductor candidate materials . Their transition temperatures could extend into the liquid nitrogen range . However , they would have Ni–Ni monolayer metallic bonds , which would limit their superconducting properties . Without a bilayer metallic bond their Tcs probably wouldn't exceed 100 K . A Valence Bond sketch of their electronic structure is presented as follows :

The electronic structure of [Fe2+O2-][Fe–Sn] would be similar to that of [Ni2+O2-][Ni–Sn] shown above , but with no electrons in the 4p orbitals . Iron (3d6 4s2) has two fewer valence electrons than nickel (3d8 4s2) , so Fe2OSn is predicted to be non-metallic (or more likely , a fairly good semiconductor , rather like gray tin) .

Here's a suggested synthesis of Ni2OxF1-xSn composites from primary chemical reagents :

(1 + x) Ni0 + ( – x) NiF2 + (1– x) Sn0 + x SnO2 ------- [HP–HT] -------> Ni2OxF1-xSn ,

where x = a mole ratio between 0 [pure Ni2FSn] and 1 [pure Ni2OSn] , taken experimentally . Three of the reactants (Ni , NiF2 , and SnO2 ) are quite refractory , so the syntheses would probably be best carried out in a high pressure–high temperature (HP–HT) apparatus , where the oxygen-sensitive metallic components can also be protected from the air . Note that while tin has a low melting point (232 C) , its boiling point is extraordinarily high (2602 C) . The liquid tin could readily diffuse throughout the reaction mixture over a wide temperature range .

All of the chemical reagents cited in this report are commercially available , eg. from Alfa-Aesar and American Elements . The low-valent copper compounds and the M2OxF1-xX series would provide a wide range of fascinating new metallic solids to examine as potential new superconductor candidate materials , some of which might have respectable (liquid nitrogen range) transition temperatures .


References and Notes


The standard redox potentials cited are versus the SHE at STP , and were from : D.R. Lide (ed.) , CRC Handbook of Chemistry and Physics , 87th edition , CRC Press / Taylor & Francis , Boca Raton (FL) , 2006 ; P. Vansek (ed.) , “Electrochemical Series”, pp. 8-20 to 8-25 . Another comprehensive reference of redox potentials is : A.J. Bard (ed.) , Encyclopedia of Electrochemistry of the Elements , various volumes , Marcel Dekker , New York , 1973-1986 . The Wikipedia web page “Table of Standard Electrode Potentials” is also useful . For a convenient tabulation of oxidizing metal oxides and their E0red values , see this GIF image (45 KB) .

French researchers : J. Padiou , D. Bideau , and J.P. Troadec , “Magnetic and Electrical Properties of Quaternary Thiospinels”, Chem. Abs. 92 , 225778j (1980) . Original article , which I haven’t been able to obtain : J. Solid State Chem. 31 (3) , pp. 401-405 (1980) .

preferential coordinations : Cotton and Wilkinson's Advanced Inorganic Chemistry provides at the beginning of each section about the various elements a tabulation of their stable valence states and the common coordinations associated with those valences . The most recent edition of this popular textbook is : F.A. Cotton , G. Wilkinson , C.A. Murillo , and M. Bochmann , Advanced Inorganic Chemistry , 6th edition , John Wiley , New York , 1999 .

chemical experience : The crystal structures of many complex cuprates , showing the metal atom coordinations , are illustrated in the review articles by H. Mller-Buschbaum : “Oxometallates with Planar Coordination”, Angew. Chem. Internat. Ed. Engl. 16 (10) , pp. 674-687 (1977) ; “The Crystal Chemistry of High-Temperature Oxide Superconductors and Materials with Related Structures”, ibid. 28 (11) , pp. 1472-1493 (1989) ; “The Crystal Chemistry of Copper Oxometallates”, ibid. 30 (7) , pp. 723-744 (1991) . The last review is the most comprehensive one .

niobium monoxide : T.B. Reed and E.R. Pollard , “Niobium Monoxide”, Inorg. Synth. 14 , pp. 131-134 , A. Wold and J.K. Ruff (eds.) , McGraw-Hill , New York , 1973 . This was reprinted in Inorg. Synth. 30 , Nonmolecular Solids , pp.108-110 , D.W. Murphy and L.V. Interrante (eds.) , John Wiley , New York , 1995 . An excellent review of the arc furnace method of inorganic syntheses with , and preparing , refractory materials : T.B. Reed , “Arc Techniques for Materials Research”, Mater. Res. Bull. 2 (3) , pp. 349-367 (1967) . Theodore Gray describes a home-made arc furnace in “Melting the Unmeltable”, Popular Science , May , 2004 ; available online here . The pellet of reaction mixture might also be fused simply by arc-welding it (a knowledgeable arc welder from a local metal fabrication shop might be employed for this) in an argon atmosphere .

Migeon and co-workers : H.N. Migeon , M. Zanne , and C. Gleitzer , “Preparation and Study of Lithium Copper Monoxide”, Chem. Abs. 84 , 172392e (1976) . Original article , which I haven’t been able to obtain : J. Solid State Chem. 16 (3-4) , pp. 325-330 (1976) .

recent Chemexplore web pages : “Antiferromagnetic Induction in High Temperature Superconductors” ; “New Layered Compounds for High Temperature Superconductivity” ; “A Rhenium Trioxide–Copper Oxide Layer Compound” ; “A Survey of Superconductors” ; “Ilmenites as High Temperature Superconductors” ; “High Temperature Superconductor Candidates Based on Modified La2CuO4 and La2NiO4”.

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

square planar coordinate covalent complexes of Ni(II) : R.S. Nyholm , “The Stereochemistry and Valence States of Nickel”, Chem. Rev. 53 (2) , pp. 263-308 (1953) ; diamagnetic square planar complexes are reviewed on pp. 273-274 .

nickel(II) complexes with weak crystal field ligands : 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 ; nickel compounds are reviewed on pp. 197-209 . See also Nyholm's comprehensive review of nickel compounds cited immediately above .

CuCl briefly became superconducting : A.P. Rusakov et al. , “Isomorphic Phase Transitions in CuCl at High Pressures”, Sov. Phys. JETP 45 (2) , pp. 380-384 (1977) [PDF , 278 KB] ; C.W. Chu et al. , “Study of Cuprous Chloride Under Pressure”, J. Less Common Met. 62 , pp. 463-467 (1978) ; C.W. Chu et al. , “Anomalies in Cuprous Chloride”, Phys. Rev. B 18 (5) , pp. 2116-2123 (1978) ; G.C. Vezzoli and J. Bera , “High-Pressure Studies of CuCl : Data on Candidacy as High-Tc Superconductor”, Phys. Rev. B 23 (6) , pp. 3022-3029 (1981) ; 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) . The following authors firmly refuted all claims of any sort of superconductivity in CuCl : E.I. Blount and J.C. Phillips , “High Temperature Superconductivity in CuCl ?”, J. Less Common Met. 62 , pp. 457-461(1978) ; B. Batlogg and J.P. Remeika , “Definite Experimental Evidence Against Intrinsic Electron-Hole Superconductivity in Pure CuCl”, Phys. Rev. Lett. 45 (13) , pp. 1126-1129 (1980) .

Geballe(1) : T.H. Geballe , “Why I Haven't Retired”, Ann. Rev. Condens. Matter Phys. 4 , pp. 1-21 (2013) [PDF , 1418 KB] .

Geballe(2) T.H. Geballe , “The Never Ending Search for High Temperature Superconductivity”, , 46 pp. , August 15th , 2006 [PDF , 697 KB] .

the presence of copper(0) : J.A. Wilson , “CuCl : Some Facts and Thoughts on High-Temperature Superconductivity”, Phil. Mag. B 38 (5) , pp. 427-444 (1978) ; J.A. Wilson and A.I. Demirel , “A Local-Pair Mechanism for High-Temperature Superconductivity at the Di -and Trivalent Sites”, Turk. J. Phys. 24 (2) , pp. 105-113 (2000) [PDF , 134 KB] .

student laboratory preparation of CuCl : H.F. Walton , Inorganic Preparations , A Laboratory Manual , Prentice-Hall , Englewood Cliffs , NJ , 1948 ; p. 154 .

Copper(I) fluoride : CuF is 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”, Advanced Inorganic Chemistry [preferred coordinations above] , p. 857 . 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 [as in McCaulay's patent : Cu0 + HF(aq) ------- (BF3 complex) ------> CuF(c) + H2 (g) ] .

copper(II) cation oxidizes the iodide : J.R. Partington , A Textbook of Inorganic Chemistry , 6th edition , Macmillan , London (UK) , 1957 : “Cupric iodide , first produced as a green precipitate , is unstable and decomposes into cuprous iodide and free iodine” (p. 725) .

DiSalvo : M.Y. Chern , D.A. Vennos , and F.J. DiSalvo , “Synthesis , Structure , and Properties of Antiperovskite Nitrides Ca3NM , M = Phosphorus , Arsenic , Antimony , Bismuth , Germanium , Tin , and Lead”, Chem. Abs. 116 , 186479e (1992) . Original article , which I haven’t been able to obtain : J. Solid State Chem. 96 (2) , pp. 415-425 (1992) .

Cava : T. He et al. , “Superconductivity in the Non-Oxide Perovskite MgCNi3”, Nature 411 (6833) , pp. 54-56 (2001) . Reported by Professor Cava's research group at Princeton University .

Solid state metathesis : J.B. Wiley and R.B. Kaner , “Rapid Solid-State Precursor Synthesis of Materials”, Science 255 (5048) , pp. 1093-1097 (1992) ; R.E. Treece , G.S. Macala , and R.B. Kaner , “Rapid Synthesis of GaP and GaAs from Solid-State Precursors”, Chem. Mater. 4 (1) , pp. 9-11 (1992) ; R.B. Kaner et al. , “Rapid Solid-State Methathesis of Refractory Materials”, U.S. Patent 5110768 , 8 pp. (May 5 , 1992) [PDF , 368 KB] ; R.B. Kaner , C.H. Wallace , and T.K. Reynolds , “Instantaneous Synthesis of Refractory Nitrides from Solid Precursors”, U.S. Patent 6096282 , 11 pp. (August 1 , 2000) [PDF , 127 KB] ; R.B. Kaner and C.H. Wallace , “Process for Rapid Solid-State Formation of Refractory Nitrides”, U.S. Patent 6120748 , 8 pp. (September 19 , 2000) [PDF , 89 KB] . Note : these three patent files can be opened only with Adobe Acrobat Reader v. 6 or later . If desired , this application can be downloaded for free from .

molten flux : T. Kimura , “Molten Salt Synthesis of Ceramic Powders”, Ch. 4 , pp. 75-100 in Advances in Ceramics – Synthesis and Characterization , Processing and Specific Applications , C. Sikalidis (ed.) , InTech , Shanghai , China (2011) [PDF , 1233 KB] . Note : this file can be opened only with Adobe Acrobat Reader v. 6 or later .

ZrCuSiAs crystal structure : R. Pttgen and D. Johrendt , Materials With ZrCuSiAs-Type Structure, Z. Naturforsch. 63B (10) , pp. 1135-1148 (2008) [ preprint , PDF , 232 KB] ; T.C. Ozawa and S.M. Kauzlarich , “Chemistry of Layered d-Metal Pnictide Oxides and Their Potential as Candidates for New Superconductors”, Sci. Technol. Adv. Mater. 9 (3) , 033003 , 12 pp. (2008) .

other Chemexplore web pages : Prediction of Superconductivity in Transition Metal Chalcogenide Oxides” ; “Electron Doping of Transition Metal Pnictides and Chalcogenides” ; “Exploring Some New Chemistry of Layered Compounds” ; “Electron-Doped Antifluorites as Superconductors.

respectable superconductor transition temperatures : The highest transition temperature to date in the LaOFeAs family of layered compounds is 56 K for for three ferroarsenides : (1) Gd0.8Th0.2OFeAs [C. Wang et al. , Thorium-Doping Induced Superconductivity up to 56 K in Gd1-xThxFeAsO”, Europhys. Lett. 83 (6) , 67006 , 4 pp. , 2008 ; preprint , PDF , 707 KB] ; (2) Sr0.5Sm0.5FFeAs [G. Wu et al. , Superconductivity at 56 K in Samarium-Doped SrFeAsF”, J. Phys. Condens. Matter 21 (14) , 142203 , 3 pp. , 2009 (PDF , 282 KB)] ; (3) Ca0.4Nd0.6FFeAs [P. Cheng et al. , “High-Tc Superconductivity Induced by Doping Rare-Earth Elements into CaFeAsF ”, Europhys. Lett. 85 (6) , 67003 , 4 pp. , 2009 (PDF , 806 KB)] .

AFM induction from the arsenic atoms : Selenium is known to induce antiferromagnetism in certain Transition metal atoms such as those of cobalt : 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 . Arsenic might have a similar AFM induction effect in certain cases , such as in the LaOFeAs family of compounds .

Kamihara and co-workers : 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) .

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

Wells : A.F. Wells , Structural Inorganic Chemistry , 3rd ed. , Clarendon Press , Oxford (UK) , 1962 . The sulfides , selenides , and tellurides of iron , cobalt , and nickel mostly have the nickel arsenide structure (Fig. 168 , p. 514) . FeSe can also have the anti-litharge structure (Fig. 160 , p. 477) .

Pure FeSe : F.-C. Hsu et al. , “Superconductivity in the PbO-type Structure a–FeSe”, Proc. Nat. Acad. Sci. 105 (38) , pp. 14262-14264 (2008) (PDF , 736 KB) ; A.J. Williams , T.M. McQueen , and R.J. Cava , “The Stoichiometry of FeSe”, Solid State Commun. 149 (37-38) , pp. 1507-1509 (2009) [ preprint , PDF , 77 KB] .

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

gray tin : A.W. Ewald and E.E. Kohnke, “Measurements of Electrical Conductivity and Magnetoresistance of Gray Tin Filaments”, Phys. Rev. 97 (3) , pp. 607-613 (1955) ; see Figure 2 , p. 609 for the graph of the electrical conductivity of gray tin over a range of temperatures . Gray tin has a respectable (for a pseudometal) electrical conductivity of 2090 ohm-1cm-1 at 273 K .



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