Exploring Some New Chemistry of Layered Compounds
The discovery in 2006 of superconductivity in the Transition metal pnictide layer compound LaOFeP sparked an explosion of feverish research activity into this class of materials . The great interest shown by solid state scientists in the previously obscure ZrCuSiAs (PDF , 2046 KB) crystal structure is comparable to that enthusiasm demonstrated by an earlier generation of researchers during the "gold rush" of the superconducting cuprates two decades ago .
(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 scope of the layered compounds has proven to be much wider than originally anticipated . The ZrCuSiAs structural family in fact encompasses an extensive range of compounds with a widely varied chemistry , as described in an excellent overview by Pöttgen and Johrendt . Most of the Transition metal elements can be used in the covalently-bonded electron acceptor layer , for example the FeP layer in LaOFeP . A note on nomenclature : contrary to common current practice , I prefer and will use throughout this web page the original designation of the layered compounds , in which the positive layer atomic components (donor layer) are written first , followed by the negative layer components (acceptor layer) . Hence , LaOFeP , i.e. [LaO]1+ [FeP]1-, not LaFePO . As can be seen in the following model of the related layer compound , LaOFeAs , the ionic and covalent layers are quite distinct and separate , with no pnictide-oxide bonds :
The oxide anions can be partly or completely replaced by fluoride anions , and as I have discussed in two related web pages (Iron and Doping) , it should also be possible to replace the pnictogen components (mainly P and As) with chalcogens (S , Se , and Te) .
The key component of the layered compounds is the acceptor (covalent) layer , since that's where the metallic bond is located ; or more specifically , in the planar sheets of Transition metal atoms , as the pnictide components are essentially just electronically inert atomic links . All of the layered compounds' covalent acceptor layers have the anti-litharge crystal structure , in which the metal atoms have a tetrahedral coordination to the pnictides , while the pnictide atoms have a tetragonal (square-based) pyramid coordination to the metal atoms . It's interesting to note that the litharge/anti-litharge crystal structure is rather rare in binary compounds ; several examples are that of litharge itself (yellow lead oxide , PbO) , SnO , and the mineral mackinawite (PDF , 434 KB) , which chemically is FeS . Most of the Transition metal pnictide and chalcogenide binary compounds have the nickel arsenide structure :
blue spheres : nickel ; brown spheres , arsenic
However , when they are combined with an electron-donating ionic layer , the metal pnictide/chalcogenide three-dimensional crystal is flattened into the two-dimensional anti-litharge layer . This turned out to be invaluable , both structurally and electronically , in making the LaOFeAs family of compounds metallic solids and superconductors .
Given the relative abundance of ZrCuSiAs-type compounds , the outlook for designing , synthesizing , and studying a profusion of new superconductor candidate materials from them seems quite promising and will doubtless engage the attention of researchers for some time to come . Looking beyond the LaOFeAs type of layered compounds , the question arises : can we fruitfully apply the insights provided by them to design entirely new donor-acceptor inorganic layer materials ? The objective of this essay is to try to answer that question by proposing several new layered structure systems that should be of interest to solid state scientists as metallic solids , and in some cases as potential superconductors .
Transition Metal Dichalcogenides as the Acceptor Layer
Several decades ago the layered Transition metal dichalcogenides were the focus of considerable research efforts , as they were found to be suitable acceptor substrates for a wide variety of chemical species , somewhat like graphite . However , while graphite could act as an acceptor for both electron donors and acceptors , the dichalcogenides were exclusively electron acceptors . The Group V/5 dichalcogenides those of V , Nb , and Ta become superconducting at temperatures near Absolute Zero , both when virgin (undoped) , and when intercalated with donor species such as organic amines and amides . The layered dichalcogenides were developed as important industrial materials as well : molybdenum disulfide , MoS2 , commonly called "moly", is a high temperature lubricant like graphite ; and titanium disulfide , TiS2 , has received much attention as the host lattice for lithium metal in lithium rechargeable batteries .
Titanium disulfide is an interesting metallic solid . It has been described as having a golden-yellow colour with a bright metallic luster . It's a fair electrical conductor , with an ambient conductivity of about 1400 ohm-1-cm-1, and it has an inverse electrical conductivity-temperature relationship from room temperature down to near Absolute Zero ; i.e. it is a True Metal (Class 2) . TiS2 has a flaky , platey morphology like graphite , consisting of three-layer "sandwiches" of sulfur-titanium-sulfur at the atomic level :
This side view of a layer of TiS2 shows the planes of titanium atoms (blue) in it , "coated" with sulfur atoms (green spheres) .
A top view of the structure is more helpful in revealing the atomic coordinations :
blue spheres : titanium (octahedral) ; green spheres : sulfur (trigonal pyramid) . This is also referred to as the cadmium iodide crystal structure .
The electronic structure of titanium disulfide has been widely studied (there is much discussion of it in the references cited in research efforts , below) . My simple analysis of TiS2 using "picture VB" provides a remarkable new view of this material , which I would like to present as follows . First , we recognize that the strong chemical bonding in TiS2 is entirely covalent in nature ; a weaker metallic bond is also present in the material . There is no ionic bonding in TiS2 . A coordinate covalent bonding model seems to best explain the observed physical and chemical properties of titanium disulfide . In this model , the two sulfur(0) atoms use all six of their valence shell electrons in the p3 trigonal pyramid hybrid AOs , leaving their 3s orbitals vacant . They donate these electron pairs into the titanium atom's empty sp3d2 inner octahedral hybrid AO :
The titanium's 4s2 valence electrons are relocated into the adjacent 3d orbitals (a relatively easy transition , since the 3d AOs in Ti are at an energy level quite close to those of the 4s and 4p orbitals) . These 3d orbitals can overlap with each other along the planes of titanium atoms (see the side view sketch of TiS2 above) to form both delta and sigma XOs . The former XO is particularly interesting because of its rarity in chemical bonding . Delta MOs and by extension , XOs are created by the face-to-face overlapping of native d orbitals :
Suppose we assign the x axis in the TiS2 crystal to the lines of Ti atoms ; then the axes perpendicular to them will be the y and z axes . The dxy,xz,yz orbitals point in between the x , y , and z axes ; that is , in between the TiS bonds , which lie along them . The yz planes will be facing each other along the chains of titanium atoms , so it might be possible to form a delta XO by continuously overlapping the 3dyz AOs along the x axis :
Note that this delta XO will be nodeless along the x axis . Such nodeless XOs can act as the metallic bond in True Metals (metallic solids having an inverse electrical conductivity-temperature relationship like the elementary metals and their alloys) . At the same time the 3dxy,xz AOs can also overlap continuously along the chain of titanium atoms , this time overlapping the ends of their lobes :
These sigma XOs will be nodal in nature ; they are periodically intersected by nodes in the electron density . Such nodal XOs are characteristic of Pseudometals , which are those metallic solids with a direct electrical conductivity-temperature relationship (like the semiconductors) .
Since the titanium atoms in TiS2 are in two-dimensional sheets , the same arguments will apply to the chains of titanium atoms extending along the y axis . There will thus be a two-dimensional delta XO throughout the layers of titanium atoms in TiS2 , permitting it to behave like a True Metal which it does . Additional electrical conductivity could be provided by the d-d sigma XOs in the Ti planes .
An alternate Valence Bond picture of the metallic bond in titanium disulfide is presented below as a note in the References section .
While TiS2 has an inverse electrical conductivity-temperature relationship and seems to be a True Metal (thus apparently using the delta XOs) , we should be cautious when interpreting the experimental conductivity data . Bernard and Jeannin pointed out that TiS2 which has been synthesized by the direct combination of its component elements is always a nonstoichiometric compound , and traces of either excess sulfur or titanium in the product can exert a strong doping effect on it . The electrical conductivity graphs of TiS2 , ZrS2 , and HfS2 , as reported by Conroy and Park (ref. : ambient conductivity) , are quite striking in this regard . That of TiS2 , as mentioned , displays an inverse electrical conductivity-temperature relationship ; the ZrS2 graph is hump-shaped , with a maximum conductivity at around 240 K ; and HfS2 has a pronounced direct electrical conductivity-temperature relationship , as does another layered dichalcogenide they investigated , SnS2 . We could attribute these conductivity observations to a predominance of either the delta or sigma TiTi metallic bonds in the various dichalcogenides , and in the case of ZrS2 , in the various temperature ranges .
It would be interesting to try to synthesize TiS2 by a method other than the combination of the two elements , so as to obtain a precisely stoichiometric compound , if possible . A metathesis reaction might be successful in this objective . Molybdenum disulfide was prepared by such a method :
MoCl5 + 5/2 Na2S -------------> MoS2 + 5 NaCl + ½ S0
This methathesis technique has also been successfully used for the preparations of WS2 , MoSe2 , and WSe2 .
The covalent precursor reagent for TiS2 would be TiCl4 , a colourless fuming liquid , b.p. 136 ºC :
TiCl4 + 2 Li2S -----------> TiS2 + 4 LiCl
The reaction could be tried with the two reagents neat , or with the Li2S first dissolved in a polar solvent like propylene carbonate , for example (then add the TiCl4 dropwise with mechanical stirring in an inert atmosphere) . Anhydrous conditions are essential , since Ti(IV) forms very strong TiO bonds with water and hydroxylic solvents . If successful , this would be an especially satisfying synthesis , as a titanium synthetic metal (with a TiTi delta XO metallic bond) will have been created from entirely molecular precursors . Note that the titanium reagent TiCl4 is produced in industrial quantities from the reductive chlorination of TiO2 , which is in turn manufactured in vast quantities (as a pigment for white paints) from the mineral ilmenite , FeTiO3 .
Metal-metal delta and sigma bonds like those illustrated above are thought to exist in certain Transition metal dimeric compounds such as molybdenum diacetate dimer :
red spheres : Mo ; green spheres : O ; the other spheres comprise the acetate ligands .
Quadruple MoMo bonds are postulated in Mo(OAc)2 : a delta bond formed from the face-to-face overlap of its 4dxy AOs with their valence electrons [Mo2+ is 4d4] , and three sigma bonds formed by the tip-to-tip overlapping of the 4d xz , yz , and z2 AOs (the 4dx2-y2 AOs are used by the dsp2 square planar hybrid AO in the coordinate covalent bonding of the Mo with the acetate ligands) . The MoMo distance in Mo(OAc)2 dimer is 2.10 Å , compared to 2.725 Å in molybdenum metal . Several other Transition metal cations can form acetate complexes isostructural with dimeric molybdenum(II) acetate :
chromium(II) ............ CrCr = 2.46 Å ........... 2.498 Å in chromium metal
copper(II) ................. CuCu = 2.64 Å .......... 2.556 Å in copper metal
rhodium(II) ............... RhRh = 2.45 Å .......... 2.690 Å in rhodium metal
The TiTi distance in TiS2 has been measured as 2.86 Å by powder X-ray diffraction ; the TiTi distance in titanium metal is 2.8956 Å at 25 ºC (a form , hcp hexagonal close-packed) . These results give some credibility to the picture of the TiTi delta and sigma bonds sketched above .
While titanium (and Zr and Hf) have four valence electrons , the Group V/5 elements V , Nb , and Ta have five , which also will be in their corresponding dxy,xz,yz AOs . Possibly this additional electron density is a significant factor in permitting their MX2 dichalcogenides to become superconducting , albeit only at temperatures close to Absolute Zero . The Group VI/6 elements Mo and W also form layered dichalcogenides , but they are gray to black diamagnetic semiconductors . This electrical behaviour would be consistent with the coordinate covalent picture presented above , as their 4,5dxy,xz,yz AOs would be filled up with the elements' six valence shell electrons .
While the coordination of the titaniums by the sulfurs in TiS2 is octahedral , in other layered dichalcogenides such as MoS2 and WS2 the coordination about the Transition metal atom is trigonal prismatic :
(the trigonal antiprism is somewhat similar to the octahedral coordination)
In such trigonal prismatic MX2 compounds the metal atom could possibly use a combination of the dp2 trigonal planar hybrid orbital and the unsymmetrical planar dsp AO to receive the chalcogen's coordinating electron pairs . This dsp+dp2 combination would be equivalent to the d2sp3 hybrid AO used in the MX2 compounds with an octahedral coordination about the M atoms :
The picture that emerges from this VB analysis is that TiS2 is actually a zerovalent compound , that is , with Ti(0) , as in the zerovalent Transition metal carbonyl compounds . Zerovalent titanium compounds are known , such as [(CH3)3P]Ti(CO)4 and [(cyclopentadienyl)Ti(CO)4]1-. The empty sulfur 3s AOs could form coordinate covalent bonds to intercalated molecules like the amines and amides , via their sp3 nitrogen lone pairs . Lithium metal atoms could also readily transfer their highly reactive 2s1 valence electrons to the TiS2 by overlapping their 2s AOs with the sulfurs' empty 3s AOs . The empty sulfur 3s AO is a reasonable explanation for why the layered dichalcogenides will readily intercalate electron donors , but not electron acceptors .
As noted above , TiS2 has the cadmium iodide crystal structure . Like the sulfurs in TiS2 the iodide anions in CdI2 also have a trigonal pyramid structure with electron pairs being donated to the cadmium cations in coordinate covalent bonds . However , iodide is 5s2 5p6 electronically , so its 5s AO is fully occupied , unlike the 3s AOs in the TiS2 sulfurs . This suggests that TiS2 and CdI2 might interact together , with the iodide 5s2 AOs overlapping continuously with the sulfur 3s0 AOs to form a layered coordinate covalent complex :
This layered adduct might have interesting metallic properties . Actually , the complexation of CdI2 and related MX2 layered compounds would be even more applicable to the Group VA/5 layered dichalcogenides , most of which are very low temperature superconductors . Many types of dopants have been crammed , jammed , and rammed into them in a futile attempt to raise their Tcs , which have remained stubbornly in the 510 K range (for example , see the organic amines and amides reference below) . A new type of layered electron donor might produce more favourable results . There are many halide compounds with the CdI2 layered structure , and many more with the closely related CdCl2 structure , that could be combined with the dichalcogenide MX2 layered compounds for investigation of their electronic properties .
The sort of electron doping rationale in the LaOFeAs family of superconductors might also be applied to TiS2 and its related layered dichalcogenides . One or two electrons could be donated to the layers by reducers . Even the Group VI/6 MX2 dichalcogenides could be used as substrates . Friend and Yoffe have pointed out (p. 42) in their review (research efforts) of dichalcogenide chemistry and physics that while "moly" MoS2 is a diamagnetic non-metal (its Mo 4d AOs are filled with six valence electrons) it can be made metallic by the intercalation in it of an Alkali metal . For example , the intercalated compounds KxMoS2 are paramagnetic metals and superconductors (Tc ~ 6.5 K) . It could be that in these cases with the filled d AOs the powerful reducer is forcing its electrons into the empty sulfur 3s AOs . Thus , in a compound like KxMoS2 the metallic bond could really be in the sulfur 3s sigma XO , and not in the Mo 4d orbitals .
Similarly , Friend and Yoffe point out that the addition of an electron from intercalated potassium in NbSe2 converts this material from a metallic solid and superconductor (Tc = 7.4 K) to KxNbSe2 , described as a "poor metal" (x < 1) and semiconductor (x = 1) . In this latter case , the added electron fills up the Nb 4d AOs , but doesn't add any to selenium's empty 4s AOs . These examples are in a general agreement with the VB picture of the layered dichalcogenides presented above for TiS2 , and can guide researchers in designing new layered materials based on the dichalcogenide acceptor layers , with a reasonable expectation of metallic behaviour in them .
The ionic layers from the doping reagents could be of two types : an MX type , with M being a reducing metal element , and X the accompanying anion (usually oxide or fluoride) ; or it could be MX2 , in which the MX2 layer is like the CdI2 complexing layer suggested above . Several examples of the layering chemistry are as follows :
½ Zn0 (m.p. 420 ºC) + ½ ZnCl2 (m.p. 283 ºC) + TiS2 ----------> [ZnCl]1+ [TiS2]1-
1/3 Al0 (m.p. 660 ºC) + 1/3 Al2O3 (m.p. 2045 ºC) + TiS2 --------> [AlO]1+ [TiS2]1-
TiO (m.p. 1750 ºC) + TiS2 --------> [TiO]2+ [TiS2]2- (Ti 3d AOs filled ; non-metallic ?)
TiO + MoS2 (m.p. 1185 ºC) --------> [TiO]2+ [MoS2]2- (sulfur 3s AOs half-filled ; metallic ?)
1/3 Al0 + 2/3 AlI3 (m.p. 191 ºC) + TiS2 --------> [AlI2]1+ [TiS2]1- ; compare with :
2/3 Al0 + 1/3 AlF3 (m.p. 1291 ºC subl.) + TiS2 --------> [AlF]2+ [TiS2]2- .
Simlarly for MoS2 and WS2 , but the Group V/5 MX2 substrates should yield non-metallic adducts for one added electron ; possibly metallic adducts could be produced with two added electrons .
Most of the products listed should be metallic solids and possibly low temperature (510 K) superconductors ; the last material , with two electrons donated to the TiS2 layers , is predicted to be non-metallic . The adduct [AlI2]1+ [TiS2]1- is especially interesting because it would have both the donated electrons and the coordinate covalent bonding , the latter with iodide anions having the p3 pyramid hybrid AOs and 5s2 electrons available for donation to the sulfurs' empty 3s AOs . Halide anions in the MX ionic layers , such as the [ZnCl]1+ layer , don't have the p3 hybrid AOs and so might not be able to coordinate with the sulfurs (or other chalcogens) in the acceptor layers . Fluoride anions are very feeble ligands ; they are like tiny Teflon™ spheres that donate little if any charge to nearby electrophilic cations . An MX2 layer such as [AlF2]1+ probably wouldn't form coordinate covalent bonds to the TiS2 sulfurs .
As discussed in the Doping web page , it's always desireable in the metallic layered compounds if the researcher is looking for any sort of superconductivity in the products to obtain a NIOS (non-integral oxidation state) electronic condition in them . This might be accomplished in this class of materials by doping the univalent compound with the "zerovalent" (undoped) analogue , as shown in the following example :
CdI2 (m.p. 387 ºC) + TiS2 ----------> [CdI2]0 [TiS2]0 ;
x [AlI2]1+ [TiS2]1- + (1-x) [CdI2]0 [TiS2]0 ----------> [AlxCd1-xI2]x+ [TiS2]x-,
where x is a mole ratio taken experimentally from 0 to 1 .
Univalent and divalent substrates could similarly be combined to provide a series of NIOS composites for testing .
While high Tc superconductors are unlikely to emerge from this latter class of metallic solids , they are nevertheless fascinating materials that I hope will receive the attention they deserve from researchers some day .
Magnesium Diboride Layered Compound Analogues
The element boron has a complex chemistry . While boron itself is nonmetallic , it can be chemically manipulated to make it metallic , as in its diboride compounds . The most famous of these is undoubtedly magnesium diboride , MgB2 . Magnesium diboride was first prepared in 1954 , and emerged from obscurity as a laboratory curiosity after it was reported to be a medium temperature superconductor (Tc = 39 K) in January , 2001 . Since then , considerable research has been carried out with MgB2 in attempts to raise its Tc .
Magnesium diboride has a layered crystal structure , somewhat like that of graphite , with hexagonal layers of boron atoms . Magnesium spectator cations are sandwiched in between :
Blue spheres : boron ; red spheres : magnesium cations .
A top view of MgB2
Boron's valence shell electrons are 2s2 2p1 . A magnesium atom can donate its two very reactive 3s2 electrons to two boron atoms (one electron each) , so that the boride anions are now 2s2 2p2 electronically . They are isoelectronic with carbon , and boron can form the same sort of hybrid atomic orbitals (described by the Valence Bond theory) as carbon , such as the trigonal planar sp2 AO and the tetrahedral hybrid AO , sp3. In the case of MgB2 the former hybrid AO is preferred , and the boride anions polymerize into a graphite-like planar sheet with hexagonal rings having BB sigma and pi bonds . The magnesium spectator cations nest in between the anionically-charged boron sheets .
Although we would normally think that the electrical conductivity in the material would be exclusively in the pi XO over the planes of boron atoms , in fact experiments have clearly demonstrated that the superconducting Cooper pairs are in two XOs , one being the boron pi XO as expected , and the second the stronger one , actually being in a sigma XO . That one has been attributed to the sigma BB bonds in the rings . It's claimed there aren't enough electrons to completely fill the sigma bonds , so there is a conduction band in them . If that was the case , I would think that the resonant one-electron BB bonds would make the compound act like a p-type semiconductor , not a superconductor , since there are nodes in the sigma bonds . Also , if there were insufficient electrons from the magnesium to complete the boron bonding , the incomplete bonds would be the higher energy pi bonds , and not the lower energy sigma bonds , which would preferentially be filled up first (Aufbau principle) .
Let's assume there actually is an incomplete transfer of reducing electrons from the magnesiums to the borons . The layers of magnesiums would then be mixed-valent (NIOS) , with a mixture of Mg0 and Mg2+. The 3s orbitals over the Mg2+ kernels could form a nodeless sigma XO , partially-filled with unused valence electrons . Perhaps it's this magnesium sigma XO that was detected in the experiments , and was erroneously attributed to the boron layers . If so , then magnesium diboride would really be a superconducting magnesium not boron compound !
The sandwich-like formation of layered compounds which was developed with the LaOFeAs family of superconductors could be applied to boron as well . The various MX reducing layers could donate two or possibly even three electrons to the boron atoms , which would become boride anions and polymerize into the planar hexagonal sheets . Several examples might include :
TiO (m.p. 1750 ºC) + 2 B0 (m.p. 2300 ºC) -----(argon , arc furnace)-----> [Ti(IV)O]2+ [B2]2-
2/3 Al0 (m.p. 660 ºC) + 1/3 AlF3 (m.p. 1291 ºC subl.) + 2 B0 ----- (argon) ------> [AlF]2+ [B2]2-
2/3 Ti0 (m.p. 1668 ºC) + 1/3 TiF3 (m.p. 1200 ºC) + 2 B0 ----- (argon) ------> [TiF]3+ [B2]3-
NIOS composites could be prepared and examined for evidence of an enhanced superconductor Tc in the products , for example :
x [AlF]2+ [B2]2- + (1-x) [TiF]3+ [B2]3- ----- (argon) ------> [AlxTi1-xF](3-x)+ [B2](3-x)-,
where x is a mole ratio taken experimentally between zero and unity .
The diboride compound with three added electrons seems unusual ; however , Russian researchers have successfully doped MgB2 with quantities of elemental rubidium , cesium , and barium , with a reported enhanced Tc in the resulting products of 52 K , 58 K , and 45 K , respectively . Where exactly the extra electrons are located in the doped materials is unknown , but they do seem to have a beneficial effect on the materials' superconducting properties . The 2 s-p to 3 s-p energy gap is quite large , so I suspect that the "extra" third electron may actually be entering a more energetically accessible ABMO (antibonding molecular orbital) associated with the BB sigma and/or pi BMOs .
The same techniques might also be applied to other hexagonal layered compounds such as hexagonal boron nitride , BN , and to AB compounds that might be forced into an isostructural material . The Zintl compound strontium digallide , SrGa2 , is isostructural with MgB2 , with Sr replacing Mg and Ga replacing B . This suggests that a gallium AB compound might be forced into a hexagonal layer in a sandwich material like LaOFeAs . Such AB gallium compounds could be GaN , GaP , GaAs , and GaSb , which are well-known as commercial semiconductors . GaN has the wurtzite crystal structure (GaP , GaAs , and GaSb have the zinc blende / sphalerite structure), but in the forcing reaction conditions of the layering process they might be flattened into hexagonal planes . For example , in GaN both the Ga and N would assume the trigonal planar sp2 hybrid AO as boron does in MgB2 , and form hexagonal sheets isostructural with those in hexagonal BN . The reducing layer would add one , two , or three electrons to the GaN :
½ Mg0 (m.p. 650 ºC) + ½ MgF2 (m.p. 1261 ºC) + GaN (m.p. 800 ºC subl.) ----- (argon) ------> [MgF]1+ [GaN]1-
½ Zn0 (m.p. 420 ºC) + ½ ZnF2 (m.p. 872 ºC) + GaN ----- (argon) ------> [ZnF]1+ [GaN]1-
2/3 Al0 + 1/3 AlF3 + GaN ----- (argon) ------> [AlF]2+ [GaN]2-
2/3 Ti0 + 1/3 TiF3 + GaN ----- (argon) ------> [TiF]3+ [GaN]3- ;
for the synthesis of NIOS composites :
LiF (m.p. 845 ºC) + GaN ----- (argon) ------> [LiF]0 [GaN]0 ;
x [LiF]0 [GaN]0 + (1-x) [ZnF]1+ [GaN]1- ----- (argon) ------> [LixZn1-xF](1-x)+ [GaN](1-x)-
x [ZnF]1+ [GaN]1- + (1-x) [AlF]2+ [GaN]2- ----- (argon) ------> [ZnxAl1-xF](2-x)+ [GaN](2-x)-
x [AlF]2+ [GaN]2- + (1-x) [TiF]3+ [GaN]3- ----- (argon) ------> [AlxTi1-xF](3-x)+ [GaN](3-x)-,
where x is a mole ratio taken experimentally between zero and unity ,
Gallium is 4 s-p electronically , so the added electrons might enter the empty 5 s-p frontier orbitals over the gallium kernels . The extra electrons would be energetically destabilized over the 2 s-p nitrogen kernels (because of the huge 2 s-p to 5 s-p energy gap) and would avoid them . Suppose two electrons were donated to three gallium atoms ; that is , only 2/3 electron per GaN formula unit :
2 Zn0 + 2 ZnF2 + 3 GaN ----- (argon) ------> [(ZnF)1+]2 [(GaN)0.67-]3
2/3 Al0 + 1/3 AlF3 + 3 GaN ----- (argon) ------> [AlF]2+ [(GaN)3]2-
TiO + 3 GaN -----(argon)-----> [Ti(IV)O]2+ [(GaN)3]2-
It might be possible to form an aromatic electron pair in the three-membered gallium ring . Such aromatic electron pairs are very stable , as in benzene and related aromatic organic molecules . We could conceive of three GaN units forming a hexagonal ring , then adding two electrons from an MX reducing layer to the ring . Would the resulting layered compound be metallic and superconducting ?
This might be a way of converting a semiconductor into a superconductor ! How stable and robust the "aromatic" Cooper pairs might be , and how high the Tc of such a layered compound might be , would be purely speculative . Nevertheless , the design , synthesis , and study of such intriguing new materials would be a worthwhile and engaging project .
References , Notes , and Further Reading
LaOFeP : Y. Kamihara et al. , Iron-based Layered Superconductor : LaOFeP, J. Amer. Chem. Soc. 128 (31) , pp. 10012-10013 (2006) .
gold rush : R.M. Hazen , The Breakthrough , The Race for the Superconductor , Summit Books , New York , 1988 . See also : B. Schechter , The Path of No Resistance , The Story of the Revolution in Superconductivity , Simon and Schuster , New York , 1989 ; J.L. Mayo , Superconductivity , The Threshold of a New Technology , TAB Books , Blue Ridge Summit (PA) , 1988 ; J. Langone , Superconductors , The New Alchemy , Contemporary Books , Chicago (IL) , 1989 ; R. Simon and A. Smith , Superconductors , Conquering Technologys New Frontier , Plenum Press , New York , 1988 .
Pöttgen and Johrendt : R. Pöttgen and D. Johrendt , Materials With ZrCuSiAs-Type Structure, Z. Naturforsch. 63B (10) , pp. 1135-1148 (2008) . This review is also available as a preprint from the ArXiv.org website (PDF , 232 KB) .
litharge/anti-litharge : A.F. Wells , Structural Inorganic Chemistry , 3rd ed. , Clarendon Press , Oxford (UK) , 1962 ; Fig. 160 , p. 477 .
nickel arsenide : Wells (above) , Fig. 168 , p. 514 .
research efforts : J.A. Wilson and A.D. Yoffe , The Transition Metal Dichalcogenides , Discussion and Interpretation of the Observed Optical , Electrical , and Structural Properties, Adv. Phys. 18 (73) , pp. 193-335 (1969) ; R.H. Friend and A.D. Yoffe , Electronic Properties of Intercalation Complexes of the Transition Metal Dichalcogenides, Adv. Phys. 36 (1) , pp. 1-94 (1987) ; A.D. Yoffe , Layer Compounds, Ann. Rev. Mater. Sci. 3 , pp. 147-170 (1973) ; A.D. Yoffe , Electronic Properties of Two-Dimensional Solids : The Layer Type Transition Metal Dichalcogenides, Festkörperprobleme 13 , pp. 1-29 in Advances in Solid State Physics , H.J. Queisser (ed.) , Pergamon Press , Oxford (UK) , 1973 ; F.R. Gamble , The Layered Dichalcogenides : Some Chemistry and Physics, Ann. New York Acad. Sci. 210 (1) , pp. 86-110 (1978) .
virgin (undoped) : L.N. Bulaevskii , Superconductivity and Electronic Properties of Layered Compounds, Soviet Physics Uspekhi 18 (7) , pp. 514-532 (1975) ; Table I , p. 517 and Table V , p. 527 .
organic amines and amides : J.F. Revelli , Tantalum Disulfide (TaS2) and Its Intercalation Compounds, Inorg. Synth. 30 (Nonmolecular Solids) , D.W. Murphy and L.V. Interrante (eds.) , John Wiley , New York , 1995 ; pp. 155-169 . See Table I , Other Molecules That Intercalate TaS2, p. 164 . Table II , p. 167 , has intercalation data for AxMX2 compounds , where A = an Alkali metal cation , and M = Groups 4 , 5 , and 6 Transition metals .
rechargeable batteries : J. Rouxel and R. Brec , Low-Dimensional Chalcogenides as Secondary Cathodic Materials : Some Geometric and Electronic Aspects, Ann. Rev. Mater. Sci. 16 , pp. 137-162 (1986) ; also discussed in the web article : anon. , Ch. 2 , Fundamentals [PDF , 1366 KB] .
ambient conductivity : L.E. Conroy and K.C. Park , Electrical Properties of the Group IV Disulfides TiS2 , ZrS2 , HfS2 , and SnS2, Inorg. Chem. 7 (3) , pp. 459-463 (1968) ; TiS2 , Fig. 2 , p. 461 .
energy level quite close : 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 .
the metallic bond in titanium disulfide : In an alternate model a different titanium hybrid orbital , the octahedral d5s , receives the donated electron pairs from the sulfur atoms . This results in titanium's 3d2 4s2 valence electrons being pushed into the 4p orbitals , which then form a nodeless pi XO as the metallic bond in TiS2 :
This is energetically possible because the 4s , 3d , and 4p energy levels in titanium are fairly close together ; the 3d energy levels are actually intermediate in energy level between those of the 4s and 4p orbitals . ARPES (angle resolved photoelectron spectroscopy , also known as ARUPS , angle resolved ultraviolet photoelectron spectroscopy) measures the energy distribution of the valence shell electrons in atoms . It would be the method of choice in differentiating between the two models of TiS2 presented on this web page : that of its metallic bond in the titanium 3d orbitals (delta XO) , or in its 4p orbitals (pi XO) .
ARPES : A.F. Santander-Syro , Introductory Lectures on Angle-Resolved Photoemission Spectroscopy (ARPES) and Its Application to the Experimental Study of the Electronic Structure of Solids [PDF , 1742 KB] ; ARPES Research Group , University of British Columbia [web page] ; A. Damascelli , Probing the Electronic Structure of Complex Systems by ARPES, Physica Scripta T109 , pp. 61-74 (2004) [PDF , 1859 KB] ; R. Comin and A. Damascelli , ARPES : A Probe of Electronic Correlations, ArXiv.org web page (ArXiv PDF , 4054 KB) ; Shen Laboratory, Stanford University , CA [web page] ; EE290f - Lecture 19 : Angle Resolved Photoemission and Non-ARPES, given by Dr. Eli Rotenberg , ALS/LBNL [YouTube video , FLV , run time 1:07:35 ; a high speed Internet connection is recommended , if not mandatory , for viewing this video !] .
Bernard and Jeannin : J. Bernard and Y. Jeannin , Investigations of Nonstoichiometric Sulfides . I . Titanium Sulfides , TiS2 and Ti2S3, Ch. 17 , pp. 191-203 in Nonstoichiometric Compounds , R. Ward (ed.) , Adv. Chem. Series 39 , American Chemical Society , Washington (DC) , 1963 .
metathesis reaction : J.B. Wiley and R.B. Kaner , Rapid Solid-State Precursor Synthesis of Materials, Science 255 (5048) , pp. 1093-1097 (1992) ; P.R. Bonneau , J.B. Wiley , and R.B. Kaner , Metathetical Precursor Route to Molybdenum Disulfide, pp. 33-37 in Nonmolecular Solids (see above in organic amines and amides) ; 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) .
propylene carbonate : I discussed this remarkable polar solvent in my web page ,A Metallic Polymer. See also : R. Jasinski , Electrochemistry and Application of Propylene Carbonate, Adv. Electrochemistry Electrochem. Eng. 8 , pp. 253-335 , P. Delahay and C.W. Tobias (eds.) , Wiley Interscience , New York , 1971 . See Table XIII , p. 279 ; W.J. Peppel , Preparation and Properties of the Alkylene Carbonates, Ind. Eng. Chem. 50 (5) , pp. 767-770 (1958) . Several physical properties of PC tabulated in this review : m.p. 49.2 ºC ; b.p. 241.7 ºC ; s.g. 1.2057 ; dielectric constant , 69.0 . Peppel notes that salts dissolved in PC can catalyze its decomposition at elevated temperatures . For example , ethylene carbonate with 1% dissolved LiCl heated at 175 ºC for 2 hours gave an estimated yield of 65% of ethylene oxide .
molybdenum diacetate dimer : F.A. Cotton and G. Wilkinson , Advanced Inorganic Chemistry , 5th edition , John Wiley , New York , 1988 ; quadruple bonds in molybdenum compounds , pp. 839-845 ; Mo(OAc)2 on pp. 840-841 (also in Table 9-C-5 , p. 840) .
acetate complexes : H. Krebs , Fundamentals of Inorganic Crystal Chemistry , transl. by P.H.L. Walter , McGraw-Hill , London , UK , 1968 ; pp. 322-323 , and Fig. 27.1 , p. 322 .
TiTi distance : A.D. Wadsley , Partial Order in the Non-Stoichiometric Phase Ti2+xS4 (0.2<x<1), Acta Cryst. 10 , pp. 715-716 (1957) . Wadsley thought there might be metalmetal bonding in TiS2 :
The TiTi distance in metallic titanium is 2.90 Å , and it is possible that some sort of metal-to-metal bonding exists between adjacent octahedra ..... (p. 716) . The TiS bond length in TiS2 is 2.45 Å .
trigonal planar : A list of many Valence Bond hybrid orbitals is provided by G.E. Kimball , Directed Valence, J. Chem. Phys. 8 (2) , pp. 188-198 (1940) ; Table XXIV , Summary of Stable Bond Arrangements and Multiple Bond Possibilities, p. 198 ; R.T. Sanderson , Inorganic Chemistry , Reinhold Publishing , New York , 1967 ; Table 8-2 , Directional Characteristics of Some Valence Orbitals, p. 112 ; and in my own ebook , Exploring the Chemistry of Metallic Solids , including Superconductors , Table 3 , Valence Bond Hybrid Orbitals, p. 64 .
Zerovalent titanium compounds : F.A. Cotton , G. Wilkinson , C.A. Murillo , and M. Bochmann , Advanced Inorganic Chemistry , 6th edition , John Wiley , New York , 1999 ; Table 17-A-1 , p. 696 .
boron : A.G. Massey , Boron, Scientific American 210 (1) , pp. 88-97 (January , 1964) ; D.R. Sullenger and C.H.L. Kennard , Boron Crystals, Scientific American 215 (1) , pp. 96-107 (July , 1966) .
first prepared in 1954 : M.E. Jones and R.E. Marsh , The Preparation and Structure of Magnesium Boride , MgB2, J. Amer. Chem. Soc. 76 (5) , pp. 1434-1436 (1954) . See also : V. Russell , R. Hirst , F.A. Kanda , and A.J. King , An X-ray Study of the Magnesium Borides, Acta Cryst. 6 , p. 870 (1953) ; A.F. Wells (litharge/anti-litharge above) , Table 130 ,The Crystal Structures of Metallic Borides, p. 825 ; L. Pauling , The Nature of the Chemical Bond and the Structure of Molecules and Crystals , 3rd edition , Cornell University Press , Ithaca (NY) 1960 ; Figure 11-15 , showing the structure of hexagonal AlB2 , p. 436 .
reported : J. Nagamatsu et al. , Superconductivity at 39 K in Magnesium Diboride, Nature 410 (6824) , pp. 63-64 (March 1, 2001) . Three excellent reviews of MgB2 as a superconductor : P.C. Canfield and S.L. Bud'ko , Low-Temperature Superconductivity is Warming Up, Scientific American 292 (4) , April , 2005 , pp. 81-87 [PDF , 855 KB] ; P.C. Canfield and G.W. Crabtree , Magnesium Diboride : Better Late Than Never, Physics Today 56 (3) , March , 2003 , pp. 34-40 [PDF , 1743 KB] ; P.C. Canfield and S.L. Bud'ko , Magnesium Diboride : One Year On, Physics World 15 (1) , January , 2002 , pp. 29-34 [PDF , 347 KB] .
experiments : S. Souma et al. , The Origin of Multiple Superconducting Gaps in MgB2, Nature 423 (6935) , pp. 65-67 (May 1, 2003) .
arc furnace : 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 Nonmolecular Solids (see above in organic amines and amides) , pp.108-110 . 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 compressed 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 .
Russian researchers : A.V. Palnichenko , O.M. Vyaselev , and N.S. Sidorov , Influence of Rb , Cs , and Ba on the Superconductivity of Magnesium Diboride, JETP Lett. 86 (4) , pp. 272-274 (2007) .
hexagonal boron nitride : Boron nitride is commercially available in two forms : cubic borazon , which has the zinc blende structure and resembles the diamond allotrope of carbon in its properties ; and hexagonal boron nitride which has been described as a soft white substance with a high melting point [~ 3000 ºC subl.] and great chemical stability" (Sullenger and Kennard , boron above , p. 102) . While isostructural with graphite it seems to be rather unreactive to the intercalation of various chemical species . Potassium has been intercalated in hexagonal BN ; the white substrate film turned violet with the added potassium metal atoms , but apparently there was little electron transfer to the BN : G.L. Doll , J.S. Speck , G. Dresselhaus , and M.S. Dresselhaus , Intercalation of Hexagonal Boron Nitride With Potassium, J. Appl. Phys. 66 (6) , pp. 2554-2558 (1989) .
Zintl compound : excellent article : H. Schafer , B. Eisenmann , and W. Müller , Zintl Phases : Transitions Between Metallic and Ionic Bonding, Angew. Chem. Internat. Ed. Engl. 12 (9) , pp. 694-712 (1973) .
strontium digallide : For example , see U. Müller , Inorganic Structural Chemistry , John Wiley , Chichester (UK) , 1993 ; Figure 65 , p. 123 .
wurtzite : A.R. West , Solid State Chemistry and Its Applications , John Wiley , Chichester (UK) , 1984 ; GaN in Table 7.9 , p. 247 (wurtzites) ; GaP , GaAs , and GaSb in Table 7.6 , p. 237 (zinc blendes) . Also in A.B. Ellis et al. , Teaching General Chemistry , A Materials Science Companion , American Chemical Society , Washington (D.C) , 1993 ; p. 146 .
hexagonal sheets : In their comprehensive review of the crystal chemistry of a wide range of inorganic nitrides , Niewa and DiSalvo discuss gallium nitride , with reference to its use as a semiconductor in electronics . They mention ,
........ hexagonal gallium nitride , GaN , a semiconductor with a wide direct band gap of ~3.5 eV , has found applications for commercial blue-light-emitting diodes (p. 2749) .
R. Niewa and F.J. DiSalvo , Recent Developments in Nitride Chemistry, Chem. Mater. 10 (10 ) , pp. 2733-2752 (1998) . This hexagonal form of GaN might be isostructural with the platey , flaky hexagonal boron nitride , BN . However , it probably occurs only as a thin film deposited by MOCVD (metal organic chemical vapour deposition) epitaxy techniques . The commercial bulk form of GaN is undoubtedly of the wurtzite variety .
aromatic electron pair : The possibility of obtaining an aromatic pair of electrons in a benzene-like molecule , with the aromatic pair above the Fermi level EF in the metallic bond sigma XO of a molecular metal , is discussed in my report , Approaching an Ambient Superconductor. The hypothetical organic molecule involved is 1,3,5-trithiabenzenium hexafluorophosphate :
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