Electron Doping of Transition Metal Pnictides and Chalcogenides
In an earlier web page I discussed the crystal and electronic structures of the compound LaOFeAs , and outlined the rationale for the design and synthesis of several new analogues in this family of metallic solids and superconductors . LaOFeAs has a layered crystal structure , consisting of a sandwich of ionic layers of LaO , having a positive charge , [La3+O2-] , i.e. [LaO]1+, alternating with layers of FeAs , having covalent FeAs bonds . The FeAs layers act as electron acceptors from the LaO donor layers ; they have a negative charge , [FeAs]1- :
The FeAs layers in the material are the electrical conduits ; the LaO ionic layers are electronically inert . A comment on nomenclature : while many authors refer to LaOFeAs as LaFeAsO that is , writing the formula as if the compound was an arsenate I have kept the original way of writing it . As you can clearly see in the sketch , there are no AsO bonds ; and I also prefer to keep the ionic layer atoms together (as LaO) and those in the covalent layers contiguous in the formula as FeAs . This is also the same for the all-fluoride compounds (eg. SrFFeAs is [SrF]1+ [FeAs]1-, not SrFeAsF) . As will become evident as this web page progresses , the design and synthesis of entirely new layer compounds is facilitated by keeping the ionic and covalent layers separated , both physically and in the mental processes of the chemist !
A key aspect of LaOFeAs is the anti-litharge crystal structure of the FeAs layers :
A side view of the litharge/anti-litharge structure , showing the planar sheets of atoms .
(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) .
I've rotated the M3D model a third time , to provide a top view of the litharge/anti-litharge structure , showing the square arrangement of the atoms .
For litharge (PbO) itself , the smaller black spheres represent the lead(II) , while the larger yellow spheres stand for the oxygen linking atoms . The small gray spheres are the inert pairs of electrons on the lead(II) . They are stereochemically prominent and result in litharge having its characteristic layered structure . For the FeAs layers in LaOFeAs , the black spheres are the arsenic linking atoms , having a tetragonal (square-based) pyramid coordination , also with a stereochemically prominent lone pair in the axial lobe . The yellow spheres are the iron atoms , aligned in planar layers in the compound . They have a tetrahedral coordination to the arsenic atoms , which we will see is important to the success of the LaOFeAs family of compounds as metallic solids and as superconductors .
The classic Valence Bond (VB) theory is helpful in describing the covalent bonds in extended lattice (nonmolecular) solids in simple , nonmathematical pictures . The electron configuration in the FeAs covalent bonds in LaOFeAs , per VB , is shown in the following sketch :
We can predict from this simple VB picture that two electrons will be located in the unhybridized , empty frontier 4p orbitals on the iron atoms . These 4p AOs are energetically readily accessible to the "extra" electrons , as iron is a 4 s-p element , and the 4p AOs are stereochemically unhindered by the arsenic atoms in the anti-litharge structure ; they point in-between the arsenics , an indispensible feature of the tetrahedral ligand environment for these layered compounds . The 4p AOs can overlap continuously over the planes of iron atoms to form a pi XO (XO = crystal orbital = the metallic bond = the conduction band . The formation of XOs in metallic solids is discussed at length in my web page ,A New Classification of Metallic Solids) .
This prediction from VB theory contradicts the results obtained by molecular orbital DOS calculations , which usually place the Fermi level , EF , in the d orbitals (PDF , 240 KB) . However , experimental support for the 4p orbitals location of the "extra" electrons and EF has recently (January , 2009) been provided by the findings of a team of researchers studying the electronic structure of an iron arsenide compound :
Here we report that the electronic structure of Ba1-xKxFe2As2 is in sharp disagreement with those band structure calculations , and instead reveals a reconstruction characterized by a (p,p) wave vector (p. 569) .
Using the analytical technique ARPES (angle-resolved photoelectron spectroscopy) , Zabolotnyy and co-workers (PDF , 495 KB) were able to prepare colour photographs of the photoelectron energy distribution in the Ba1-xKxFe2As2 lattice (Tc = 38 K at x = 0.4 , PDF , 750 KB) . Analysis of graphs plotted from these photos seemed to show the electron distribution around the iron atoms is in p-type functions , and not in the anticipated d orbitals .
Although LaOFeAs is a metallic solid , it fails to become superconducting at any temperature . The related pnictide , LaOFeP , does become superconducting at Tc ~ 4 K . Doping LaOFeAs with fluoride anions that is , replacing some of the oxide anions in the LaO layers with fluorides resulted in the compound LaO0.89F0.11FeAs having a superconducting Tc = 26 K . Fluoride doping has the chemical effect of electron doping , or adding electrons to the iron atoms , specifically into their 4p AOs :
The reverse process , that of "hole doping" in essence , removing electrons from the iron atoms' 4p AOs can be accomplished by cation doping LaOFeAs . Experiments in which some of the La3+ was replaced by Ca2+ failed to produce any superconducting compounds :
Further , Tc was not observed for Ca2+-doped samples , suggesting that a critical factor for induction of superconductivity is electron doping , and not hole doping (Kamihara et al. , p. 3297) .
In a recent communication after this web page had been posted to Chemexplore , H.H. Wen advised me that cation doping can indeed successfully induce superconductivity in LaOFeAs . The strontium-doped composite La0.87Sr0.13OFeAs had a Tc ~ 25 K . The researchers concluded that ......conduction in this type of material is dominated by hole-like charge carriers , rather than electron-like ones. A similar result was obtained in the series of cation-doped Pr1-xSrxOFeAs composites , where a maximum Tc of 16.3 K was obtained at doping levels of x = 0.20 ~ 0.25 . So apparently downward cation-doping with lower-valent cations in the ionic layer , effectively removing electrons from the FeAs layers will also enable the emergence of superconductivity in the ferropnictides . Note carefully , though , that the FeAs layers in Wen's doped composites La0.87Sr0.13OFeAs and Pr1-xSrxOFeAs have a NIOS (non-integral oxidation state) valence condition , and I suspect that it's this important and possibly crucial aspect of the ferropnictide compounds , with respect to their superconductivity , that was responsible for the appearance of superconductivity in them . The doping of MOFeAs and MFFeAs compounds to obtain a NIOS condition in their FeAs layers will be discussed in greater detail further down this web page .
Of course , that begs the question : why didn't Kamihara et al. (Professor Hosono's research group) observe the same superconductivity in their calcium-doped LaOFeAs ? Unfortunately they didn't provide very much information about this aspect of their study (ref. Kamihara et al.) .
All subsequent experimental results with a profusion of LaOFeAs analogues have supported this early observation . Replacement of the original lanthanum rare earth cation with others has succeeded in raising the superconducting Tc ; for example , the compound CeO0.86F0.14FeAs had a Tc = 41 K (Chen et al.) , and the fluoride-doped samarium iron arsenide SmO0.9F0.1FeAs superconducts at Tc = 55 K (Ren et al. , PDF , 713 KB) . The researchers in this latter case noted that ,
....... the Tc is observed to be increased by the smaller rare earth substitution with shrunk crystal lattice (p. 2) .
This same "chemical trick" has been utilized in studies of the YBCO analogues two decades ago . It is usually referred to as chemical pressure, which I discussed and named the Hagenmuller effect in my ebook (pp. 380-383) after the prominent French solid state chemist , Professor Paul Hagenmuller of the Université de Bordeaux . Most notably it was the critical design feature in the synthesis of YBCO , the first genuine "high temperature superconductor", i.e. in the liquid nitrogen (77 K) range of temperatures . For example , compare YBCO's lattice constant and yttrium's crystal ionic radius with those of its isostructural lanthanum analogue , LaBa2Cu3O7 :
YBa2Cu3O7 ... a = 3.86 Å ; crystal ionic radius of Y3+ = 0.90 Å ; Tc = 93 K
LaBa2Cu3O7 .. a = 3.94 Å ; crystal ionic radius of La3+ = 1.03 Å ; Tc = 59 K
There is an excellent linear relationship between the crystal ionic radius , per Shannon-Prewitt , of the rare earth cation in the YBCO series of analogues and their respective transition temperatures . It's not surprising to see the Hagenmuller effect show up once again in the LaOFeAs family of compounds .
The samarium rare earth cation , Sm3+, has a crystal ionic radius of 0.96 Å . Would replacing some or all of it with Y3+ (0.90 Å) in the series MOxF1-xFeAs raise the material's Tc a few kelvins more ? This experiment has been carried out by Italian researchers . They found that compared to the Hosono group's LaO0.89F0.11FeAs with Tc = 26 K , the 50% yttrium-substituted analogue La0.50Y0.50O0.85F0.15FeAs had a maximum Tc = 40 K .
It would also be interesting to see if a small non rare earth trivalent cation such as Al3+ (crystal ionic radius , six-coordinate , 0.54 Å) could be used as the "M" cation :
1/3 Al0 + 1/3 Al2O3 + ½ Fe0 + ½ FeAs2 --------> AlOFeAs
The formal electron doping compound AlO , i.e. [Al3+ O2- (e-)] , is unknown at room temperature , although it's thought to have a transient existence at high temperatures , such as would be used in the preparation of the hypothetical compound AlOFeAs . Fluoride-doped derivatives of AlOFeAs could also be synthesized and studied :
1/3 (2-x) Al0 + 1/3 x Al2O3 + 1/3 (1-x) AlF3 + ½ Fe0 + ½ FeAs2 --------> AlOx F1-x FeAs
In this series of fluoride-doped compounds , x is a mole ratio taken experimentally from 0 to 1 by the researcher . If a quantity of the pure precursor FeAs is available , it could replace the ½ Fe0 + ½ FeAs2 combination . The reagent FeAs2 , which is commercially available , is suggested as a non-volatile source of arsenic for the synthesis of the transient intermediate , FeAs . If x = 0 in the above oxide-fluoride composites , the pure compound AlFFeAs would be produced . All-fluoride ferroarsenides are discussed later in this web page .
In the Iron web page I pointed out the possibility of using tetravalent "M" elements in the MOFeAs series as a way of electron doping the FeAs layers . Two such M elements are uranium(IV) , with a crystal ionic radius , per Shannon-Prewitt , of 1.00 Å (8-coordinate , in the fluorite UO2 lattice) , and the more familiar titanium(IV) , with a much smaller radius of 0.61Å (6-coordinate) . Titanium(II) oxide , TiO , could be used as the reducing agent , adding two electrons per formula unit to the FeAs :
TiO + FeAs -------(argon atmosphere)-------> TiOFeAs , i.e. [Ti(IV) O2-]2+ [FeAs]2- .
TiO is commercially available as a very fine (325 mesh) powder . It's a refractory material (m.p. 1750 ºC) having a bronze colour and metallic luster and is an electrical conductor . Then , TiOFeAs could be fluoride-doped , adding even more electrons to the layers of iron atoms :
x TiO + 2(1-x)/3 Ti0 + (1-x)/3 TiF3 + ½ Fe0 + ½ FeAs2 -----------> TiOx F1-x FeAs
Again , x is a mole ratio taken experimentally between zero and unity , with the researcher preparing as many doped composites as are considered desirable and necessary . The compound titanium(III) fluoride , TiF3 , a violet-coloured crystalline solid melting at around 1200 ºC , is also commercially available , as are grades of very fine titanium metal powders .
It would also be very interesting to prepare TiOFeAs composites in which there is a non-integral charge on the FeAs layers (this is discussed in more detail below) . It seems that the LaOFeAs family of compounds including the all-fluoride analogues such as SrFFeAs are adversely affected by a spin-density wave (SDW) if there is an integral charge on the FeAs layers . This SDW apparently inhibits the formation of Cooper pairs and the appearance of superconductivity in the materials . The solution to the problem is to ensure a non-integral charge is imposed on the FeAs layers . This can be readily accomplished experimentally by the researcher . In this case , the substrate TiOFeAs (integral , x = 2) can be doped with AlOFeAs (integral , x = 1) , so that the charge on the FeAs layers in the composite lies between 1 and 2 (non-integral oxidation state , NIOS) :
x [AlO]1+ [FeAs]1- + (1-x) [TiO]2+ [FeAs]2- -----------> [Alx Ti1-x O](2-x)+ [FeAs](2-x)-,
where x is a mole ratio taken experimentally between zero and unity by the researcher .
The pure reagent titanium(III) oxide , Ti2 O3 , is commercially available as a 325 mesh , violet-coloured powder , m.p. 2130 ºC (dec) . It might be used in combination with TiO to produce a NIOS form of TiOFeAs :
x TiO + (1-x)/2 Ti2 O3 + ½ Fe0 + ½ FeAs2 -------(argon)-------> [TiO(3-x)/2 ](1+x)+ [FeAs](1+x)-.
Again , x = 0 to 1 . Note the slight molar excess of oxide anion in the composites .
The current (June , 2009) record holder for the highest transition temperature in the LaOFeAs family of superconductors is the compound Gd0.8Th0.2OFeAs , with Tc = 56 K . Note the use by the researchers , Wang and coworkers (PDF , 707 KB) , of thorium(IV) in the donor layer . This is the only example I could find of a tetravalent element used in the ionic layer of a LaOFeAs compound . Wang et al. observed a Tc with thorium doping 20 K higher than for the fluoride-doped composite GdO0.83F0.17FeAs (Tc = 36 K) . They found that cation electron doping was easier experimentally than anion (fluoride) electron doping :
...... doping F- (or oxygen vacancy) in GdFeAsO is particularly difficult , which is probably the main obstacle to elevate Tc . Substitution of Ln3+ by relatively large tetravalence ions is an alternative route to introduce electrons (p. 1) ; and ,
Moreover , the electron-doping is more easily induced by the Th4+-substitution compared with the F- substitution in GdFeAsO . This manifests that the lattice match between Ln2O2 fluorite-type layers and Fe2As2 antifluorite-type layers is important not only for the chemical stability of the parent compounds , but also for the electron-doping level in LnFeAsO . It is thus expected that the thorium-doping strategy can be applied to other iron-based oxypnictides (pp. 2-3) .
One problem that could be encountered with fluoride doping of oxides is that of "mixed phases". Several researchers in the late 1980s experimented with fluoride doping of superconducting ternary copper oxide compounds , with unverified claims of Tc enhancements of up to 155 K . Apparently there were problems with "mixed-phases" in these materials ; it seems the fluoride chemicals didn't blend in smoothly with their oxide counterparts , and complex mixtures were formed . Consideration might therefore be given to an all-fluoride composition such as TiFFeAs , i.e [Ti(IV)F]3+ [FeAs]3-. In this last example three electrons per formula unit will have been donated to the FeAs layers :
2/3 Ti0 + 1/3 TiF3 + ½ Fe0 + ½ FeAs2 -----------> TiFFeAs
The all-fluoride layer compound SrFAsFe has been prepared and studied ; its synthesis was reported almost simultaneously , in mid-October of 2008 , by M. Tegel et al. (PDF , 1145 KB) and F. Han et al. (PDF , 661 KB) . SrFAsFe has the same crystal structure as LaOFeAs (sketch above) . The SrF layer is donating only one electron to the FeAs per formula unit , [SrF]1+ [FeAs]1-, as does LaO in LaOFeAs , a metallic solid that doesn't become superconducting . When SrFAsFe is cooled down , its electrical conductivity also declines until about 175 K . Below that temperature , the conductivity of SrFAsFe begins to rapidly rise , but it still doesn't become superconducting . It was thought that magnetic ordering associated with a spin density wave (SDW) occurred at that transition temperature , and somehow inhibited the appearance of superconductivity .
The apparently anomalous SDW behaviour of SrFFeAs around 175 K can be rationalized by the simple VB picture presented in the sketch above for LaOFeAs . Let's assume that the two "extra" electrons actually are located in the iron atoms' 4p AOs , as predicted by VB . Those electrons in the 4pz AOs can participate in the metallic bond pi XO , as they overlap continuously in the x-y plane like the pi XOs in graphite and MgB2 . The "extra" electrons in the 4px and 4py AOs , in addition to forming pi XOs in the x-y plane , can simultaneously form FeFe sigma covalent bonds as their p AOs overlap end-to-end :
As discussed in my Solids web page , the pi type of XOs are nodeless along two crystal axes , and metallic solids with them have an inverse temperatureelectrical conductivity relationship (I called them "True Metals", like the common elementary metals and their alloys) . The sigma type of XOs have periodic nodal spacings around the atomic kernels , resulting in the material having a direct temperatureconductivity relationship (the "Pseudometals", like the semiconductors) . X. Zhu et al. (PDF , 521 KB) also studied the electrical conductivity of SrFFeAs (and CaFFeAs and EuFFeAs) from near Absolute Zero to 300 K . Their findings , which were published on the ArXiv.org website several weeks later than those of Tegel and co-workers , confirmed the latter group's results with SrFFeAs , including its SDW transition temperature of about 175 K :
This sketch is a re-working of Zhu and coworkers' graph for SrFFeAs in their Fig. 2 , p. 9 . I replotted their original resistivity data points in the more familiar (to me) conductivity terms , versus the absolute temperature of the sample . It's quite similar to Tegel and co-workers' electrical resistivity graph for their sample of SrFFeAs . Actually , two transition points are seen on the graph : the original one near 175 K , and a lower temperature one at about 90 K .
There seems to be a complex shifting of the metallic bond in SrFFeAs as it's cooled down toward Absolute Zero . In one temperature range the nodeless pi XO is favoured ; in another temperature region the nodal sigma XO predominates . Perhaps there is a mixture of both sigma and pi FeFe bonds below 175 K , as suggested by other physical analyses of the material . Tegel et al. observed a distortion of its FeFe bond lengths from one length at room temperature , to two slightly different lengths below 175 K . Their analysis of the compound by Mössbauer spectroscopy also clearly demonstrated the "separating out" of two distinct types of iron atoms as it was cooled down toward Absolute Zero . This electronic behaviour is a clear indication of disproportionation in the material , and has been demonstrated (also by Mössbauer spectroscopy) in hopping semiconductors such as Eu3S4 . The lattice dimensions and FeFe bond lengths are shrinking as the crystal is cooled , and the shortening bonds may also be affecting the magnetic spins of the 4p "extra" electrons .
Wu and co-workers (PDF , 282 KB) discovered that by doping SrFFeAs with Sm3+ the SDW could be suppressed and the resulting composite had a simple inverse temperatureconductivity relationship (a True Metal) over a wide temperature range . At a 50 : 50 mole ratio of Sr2+ : Sm3+ the composite Sr0.5Sm0.5FFeAs had a sharp Tc = 56 K , an excellent result demonstrating the efficacy of the electron doping method in raising the substrate's Tc . In orbital terms the iron atoms' 4p pi XO seems to completely predominate in this SrSm composite .
In a recent communication after this web page had been posted to Chemexplore , H.H. Wen advised me that his research team had also synthesized an all-fluoride ferropnictide having Tc = 56 K ; this was the doped composite Ca0.4Nd0.6FFeAs . The related compound Ca0.4Pr0.6FFeAs also was superconducting at Tc = 52 K . Conduction in these compounds was thought to occur via electron-like carriers , although it was by hole-type carriers with the undoped parent compound CaFFeAs .
Clearly , for all-fluoride compositions trivalent and tetravalent M elements should be used , so that two and three electrons respectively will be donated to the FeAs layers . Then superconductivity at a respectable Tc can be anticipated in the products . The following Table lists a number of possible M candidate elements , together with their trivalent (or tetravalent , for ThF4) fluorides , that might be investigated as electron donors for the FeAs layers :
In the above Table , the "electrons" refer to the number of electrons per formula unit donated by the M metal to the FeAs layers . It will always be one less than the valence of the stable (lowest energy redox state) M ionic/covalent species in the MF donor layer . The zerovalent M metal powder , together with its corresponding fluoride salt and the FeAs (or FeAs precursors) , are ground together and heated in an atmosphere of pure , dry argon ; for example :
2/3 Al0 + 1/3 AlF3 + ½ Fe0 + ½ FeAs2 ----- (argon) ------> AlFFeAs
2/3 Ti0 + 1/3 TiF3 + ½ Fe0 + ½ FeAs2 ----- (argon) ------> TiFFeAs
¾ Th0 + ¼ ThF4 + ½ Fe0 + ½ FeAs2 ----- (argon) ------> ThFFeAs
Blends of trivalent M metals and the tetravalent titanium or thorium could be investigated , in which a variable number of electrons , from 2 to 3 , are increasingly donated to the FeAs layers :
x [AlF]2+ [FeAs]2- + (1-x) [TiF]3+ [FeAs]3- -----------> [Alx Ti1-x F](3-x)+ [FeAs](3-x)-
As usual x is a mole ratio taken experimentally from zero to unity , with the preparation of as many composite specimens as the researcher considers appropriate . Because the iron atoms are covalently bonded and aren't discrete cations , the mixed-valent state will be in the FeAs layers . By adding "odd" numbers of electrons to the FeAs layers a non-integral oxidation state (NIOS) can be established in them . This NIOS condition may in fact be critical for the appearance of superconductivity , and for the optimization of Tc in the composites . For example , no superconductivity is observed in "pure" compounds such as LaOFeAs , SrFFeAs , CaFFeAs and EuFFeAs , whose FeAs layers have integral oxidation states of 2- or 3- . To date , the only superconductivity detected in IOS layered pnictides has occurred in LaOFeP and LaONiP (for both Tc ~ 35 K) . The compound Sr2VO3FeAs , recently reported by H.H. Wen's research team , while apparently IOS (it may contain mixed-valent vanadium) , has a remarkably high Tc of 37.2 K . In all other cases either anion (fluoride) or cation [Th(IV)] doping has been necessary for superconductivity in the material . A NIOS condition is found in the iron layers of , and was undoubtedly necessary to achieve the relatively high Tc values in , the electron-doped materials SmO0.9F0.1FeAs (55 K) , Gd0.8Th0.2OFeAs (56 K) , Sr0.5Sm0.5FFeAs (56 K), and Ca0.4Nd0.6FFeAs (56 K) . A reasonably high Tc might occur in the aluminum/titanium composites [Alx Ti1-x F](3-x)+ [FeAs](3-x)- , where (3-x) is non-integral , with the experimental mole ratio x taken in steps between zero and unity , so that (3-x) lies somewhere between 2 and 3 .
As to why a NIOS condition in the iron atoms is necessary for superconductivity , I interpret the spin density wave (SDW) in the IOS materials as being simply the periodic localization disproportionation of the iron atoms' 4p electrons in FeFe bonds . Because of their even number in the IOS materials , these electrons will tend to magnetically pair together in localized MOs . In a NIOS composite there will be an odd number of 4p electrons . The unpaired electrons will resonate over the planes of iron atoms ; this electron resonance will prevent the SDW from forming .
An excellent example of a NIOS metallic material is KCP , the platinum coordinate covalent complex K2Pt(CN)4Br0.3 . 3H2O . The precursor to KCP is potassium tetracyanoplatinite , K2Pt(CN)4 , which is a rather ordinary salt-like substance , coloured lemon-yellow and water-soluble , not at all metallic . Its platinum(II) atoms have two electrons in their 5dz2 AOs , which is an IOS condition . The oxidizer bromine removes those two electrons from about one-fifth mole of Pt(II) , producing Pt(IV) . The 5dz2 AOs on the Pt(II) and Pt(IV) overlap , and KCP crystallizes in coppery metallic needles . The chains of platinum atoms in the molecular stacks are now in a NIOS condition ; KCP is formally a Robin-Day Class IIIB mixed-valent compound . The electrons can resonate throughout the Pt chains , making the compound a metallic solid (although it's a Pseudometal , because of the periodic nodes around the platinum atoms) . The electrons are reproportionated evenly dispersed over the platinum atoms at higher temperatures , resulting in a NIOS valence on the platinums of about 2.3+ . When KCP is cooled down , the 5dz2 electrons gradually "settle down" on their parent platinum kernels , and disproportionation causes the original IOS Pt(II) and Pt(IV) components to re-appear . When KCP becomes IOS again , the metallic bond and electrical conductivity both disappear in it .
The all-fluoride compounds have two advantages over the corresponding oxide materials . First , using fluoride instead of oxide provides an additional doping electron that would otherwise be "wasted" on the inert oxide anion . For example , compare LaO = [La3+O2-(e-)] with LaF = [La3+F-(2e-)] . Second , the trivalent metal fluoride precursor compounds are generally less refractory than the corresponding oxides , so a lower furnace temperature can (and should) be used in the syntheses . The commoner muffle furnace , operating at around 10001100 ºC , should be acceptable for use with the all-fluoride compounds , while the oxide-based products should preferably be fused in an arc furnace at 2000+ ºC .
The perceptive reader will have noted the suggested use of three paramagnetic Transition metal fluorides in the "Practical Metals and Fluorides" table above . The Transition metal fluorides are well-known for having a pronounced Curie paramagnetism . A number of the simpler fluorides are antiferromagnetic (although with quite low Néel temperatures) , and many of the more complex metal fluorides are ferrimagnetic . This paramagnetism occurs in them because fluoride is a very feeble nucleophile and ligand in coordinate covalent bonding , being well down near the "weak end" of the Spectrochemical Series of ligands . As a result , the nd orbital valence shell electrons in the Transition metal fluorides are all in a high spin (spin free) condition , which is reflected in the relatively high magnetic susceptibilities of these compounds :
The inclusion of one of these paramagnetic Transition metal fluorides in the MF donor layer would be an interesting test of the concept that it's quite possible to have paramagnetic cations in a functional superconductor , so long as they aren't involved in the metallic bond ; that is , they should be "off to the side" of it . There are several examples of this from the cuprate high Tc superconductors (also note the Hagenmuller effect) :
DyBa2Cu3O7 .. Dy3+ is 4f9 electronically ; its crystal ionic radius is 0.91 Å ; Tc = 92 K ;
GdBa2Cu3O7 .. Gd3+ is 4f7 ; radius = 0.94 Å ; Tc = 91 K ;
SmBa2Cu3O7 .. Sm3+ is 4f5 ; radius = 0.96 Å ; Tc = 87 K ; and ,
NdBa2Cu3O7 .. Nd3+ is 4f3 ; radius = 0.98 Å ; Tc = 78 K .
In the cuprates the metallic bond is the pi and sigma-pi XO over the copperoxygen covalent bond skeleton . The rare earth trivalent cations are nested in between the CuO layers , and they apparently aren't electronically or magnetically coupled to the metallic bond XO . As a result , the Cooper pairs can co-exist with them as they form in the XO .
In the ferroarsenides the paramagnetic cations in the fluoride (or oxide) layers theoretically shouldn't inhibit the formation of Cooper pairs in the FeAs layers .......... or would they ? It would be a very interesting experiment ! A series of NIOS composites could be designed and synthesized for a test of such a deliberate paramagnetic "contamination" of a superconductor ; for example :
2/3 Cr0 + 1/3 CrF3 + ½ Fe0 + ½ FeAs2 ----- (argon) ------> CrFFeAs ;
2/3 Ti0 + 1/3 TiF3 + ½ Fe0 + ½ FeAs2 ----- (argon) ------> TiFFeAs ; then ,
x [CrF]2+ [FeAs]2- + (1-x) [TiF]3+ [FeAs]3- ----- (argon) ------> [Crx Ti1-x F](3-x)+ [FeAs](3-x)- ,
with x being a mole fraction taken between zero and unity .
It would be quite extraordinary to obtain a superconductor that is known to have a significant content of Transition metal cations with a strong Curie paramagnetism , such as chromium(III) .
Other Transition Metals in Ferropnictide Layered Superconductors
Iron and nickel have been used in Transition metal pnictide layer superconductors , and cobalt has been employed as a low level dopant in several types of ferropnictides (for example , in SrFFeAs , as described by S. Matsuishi et al. , Cobalt-Substitution-Induced Superconductivity in a New Compound with ZrCuSiAs-type Structure , SrFeAsF, PDF , 381 KB) . These researchers were able to obtain a maximum Tc = 4 K for the doped composite SrFFe0.875Co0.125As ; higher or lower cobalt doping failed to induce superconductivity in the substrate .
Matsuishi and co-workers (Professor Hosono's group) commented that cobalt was doping the FeAs layers with electrons :
The decrease in the c-axis indicates the enhanced Coulombic interaction between the (SrF)+ and (FeAs)- with the Co content , providing evidence that the Co-substitution adds excess electrons to the (FeAs)- layers.
Doping CaFFeAs with cobalt produced the composite CaFFe0.9Co0.1As with a somewhat higher Tc = 22 K . The smaller Ca2+ cation (1.00 Å) compared to the Sr2+ cation (1.18 Å) may have induced the Hagenmuller effect (shrinking the crystal lattice) in the compound and raised its Tc .
Quite possibly in the anti-litharge crystal structure the arsenic atom ligands are pressing down on the cobalt atoms , "popping" [promoting] their 3d7 valence shell electrons into the 4p frontier orbitals . Such a promotion should be energetically feasible , since the 3d orbitals in cobalt lie , in energy terms , very close to the 4s and 4p AOs , and the 4p orbitals are stereochemically unhindered by the arsenic atoms (litharge/anti-litharge sketches above) :
Scrolling back up this web page , you'll see that the hypothetical compound SrFCoAs should have the same electronic structure as fluoride-doped LaOFeAs (if its CoAs acceptor layer has the anti-litharge structure) ; the compound LaO0.89F0.11FeAs has a Tc = 26 K . The pure material SrFCoAs might be prepared in the usual manner :
½ Sr0 + ½ SrF2 + CoAs ----- (argon) ------> SrFCoAs
The required reagents (strontium pieces , <6 mm , 99.95% , m.p. 770 ºC ; anhydrous strontium fluoride , powder , 99.95% , m.p. 1473 ºC ; and cobalt arsenide , CoAs , -10M powder , 99.5%) are all commercially available . Other metal fluoride donor layers could be investigated as well , such as with AlF , TiF , ThF , and so on , as discussed above . Then , composite compounds with two different MF donor layers could be synthesized , having NIOS charges on the layers ; for example :
x [AlF]2+ [CoAs]2- + (1-x) [TiF]3+ [CoAs]3- ----- (argon) ------> [Alx Ti1-x F](3-x)+ [CoAs](3-x)-
The mole ratio x would be taken experimentally beween zero and unity so that the value (3-x) would vary between 2 and 3 , giving a series (perhaps a half dozen or so composites) of compounds whose layers deliberately have been given a NIOS condition . This should permit an optimization of the superconductor Tc in the various series of new materials .
There are many quaternary compounds with the ZrCuSiAs crystal structure (PDF , 2046 KB) ; a comprehensive list is provided in the excellent review of these materials by Pöttgen and Johrendt . Most of the Transition metals , including the platinum group metals , have been included in their electron acceptor (covalent) layers . Use of these Transition metal elements in new layered oxides and fluorides could provide a rich source of pnictide and chalcogenide superconductor candidates . However , it would be helpful to try to predict the possible electron configuration in the Transition metal component of the compounds before actually trying to fabricate them into superconducting materials in the laboratory .
The oxide LaOCuS and fluoride EuFCuS are both known (see Pöttgen and Johrendt's review for their references) . By valence counting the former compound has Cu(I) : [La3+O2-] [Cu1+S2-] . Such Cu(I) electronically is 3d10 , and indeed LaOCuS is diamagnetic as expected . The europium fluoride layer compound should have Cu(0) , which is 3d10 4s1. Where is that "extra" 4s1 electron ? The prediction from a simple picture VB analysis suggests that it might be in the coppers' 5p AOs :
The copper atoms , covalently bonded to the sulfur atoms , are tetrahedrally coordinated by them . Since the copper's 3d valence shell is filled , it will use the p-block tetrahedral hybrid orbital , sp3, to form the necessary CuS covalent bonds in the structure . The sulfur atoms will use the square pyramid sp3d hybrid orbital , but to complete the four CuS bonds they must use all six of their valence shell (3s2 3p4) electrons to fill the four pyramid "side lobes" and leave the fifth axial lobe empty . With this electronic arrangement the compound LaOCuS has no unpaired electrons and is predicted to be diamagnetic , in agreement with experimental data (as noted in Table 2 , p. 31 of Pöttgen and Johrendt's review) .
In the EuFCuS case , "EuF" will donate two electrons to the CuS layer . One electron will be used in the CuS covalent bonds , as with LaOCuS . The second electron will enter a suitable empty frontier orbital above the covalent bond skeleton . The tetrahedral clustering of sulfur atoms around the copper atoms will at least partially destabilize the copper 5s AOs ; however the 5p orbitals extend in between the sulfur atoms (litharge/anti-litharge sketch above) and are stereochemically unhindered by them . They are also at about the same energy level as the 5s orbitals , so they should be accessible to the donated "extra" electron from the EuF layer .
In this simple Valence Bond picture the compound LaOCuS is predicted to be non-metallic (or at best semiconducting) , while the fluoride analogue EuFCuS is predicted to be metallic . If it is expected to be superconducting it will require more donated electrons and be in a NIOS condition :
2/3 Al0 + 1/3 AlF3 + CuS ----- (argon) ------> AlFCuS ;
2/3 Ti0 + 1/3 TiF3 + CuS ----- (argon) ------> TiFCuS ; then ,
x [AlF]2+ [CuS]2- + (1-x) [TiF]3+ [CuS]3- -----------> [Alx Ti1-x F](3-x)+ [CuS](3-x)-.
As a practical measure I've substituted the abundant and cheap aluminum for the rare and costly europium ; and in any case , the smaller Al3+cation (0.54 Å) might induce a beneficial Hagenmuller effect in the doped composites compared to the larger Eu3+cation (0.95 Å) . The undoped substrates AlFCuS and TiFCuS , while metallic solids , aren't expected to be superconducting . However , if the mole ratio x in the doped composites is carefully chosen so that the sum (3-x) lies between 2 and 3 , the overall electronic charge on the copper layers will be non-integral , and these NIOS compositions may indeed be superconducting . Since the compounds EuFCuS and AlFCuS and their derivatives can be thought of as copper-based synthetic metals , i.e. having direct CuCu metallic bonds (a pi XO) , having them superconducting would be a nice accomplishment , since copper metal itself never becomes superconducting , even very close to Absolute Zero .
The same sort of analysis can be applied to layered materials with zinc as the metal atom component in the electron acceptor layers . Let's consider the hypothetical compound TiFZnS :
The Valence Bond analysis of TiFZnS is similar to that of EuFCuS immediately above . The "TiF" electron-doping layer has donated three electrons to the zinc sulfide substrate . Normally , ZnS has either the wurtzite or zinc blende (sphalerite) crystal structure , with both the zinc and sulfur atoms tetrahedrally coordinating each other . The "TiF" forces the ZnS into sandwich-like layers , and thereby adopt the anti-litharge structure in which the sulfur atoms have a flattened square pyramid coordination to the zinc atoms (which retain their tetrahedral coordination by the sulfurs) .
The sulfur atoms have six valence shell electrons in the 3 s-p native orbitals , and they can readily form a square pyramid hybrid sp3d AO , with five bonding lobes : four on the sides of the pyramid , and an axial lobe with the nonbonding lone pair of electrons . The zinc atoms' 4 s-p valence shell orbitals form the usual tetrahedral sp3 hybrid AO ; then they "polymerize" with the sulfur orbitals to create the covalent bond skeleton of the anti-litharge structure . The "TiF" reducing layer provides the two additional electrons needed to complete the strong , low energy ZnS covalent bonds .
The third "TiF" electron is leftover after this bond formation . It will enter any accessible empty frontier orbitals on the zinc atoms , which are predicted to be the 5p AOs . The 5s orbitals may be partially destabilized by the four sulfurs clustering around each zinc ; however , as has been pointed out earlier , the p AOs in a tetrahedral ligand environment are stereochemically unhindered , as they point in between the ZnS bonds , a very useful feature of the anti-litharge structure . Thus , TiFZnS is predicted to be a metallic solid . It will undoubtedly be affected by the spin density wave (SDW) phenomenon found in all of the anti-litharge ZrCuSiAs-type materials . Unfortunately , there is no chemical system that might dope the substrate ZnS with four electrons . The corresponding compound which has been doped with only two electrons , for example with "AlF", would have to be used with TiFZnS to prepare NIOS composites in which the SDW is suppressed and superconductivity might appear . This latter compound , AlFZnS , lacks the "extra" electron in the zinc 5p AOs , and so is predicted to be non-metallic :
2/3 Al0 + 1/3 AlF3 + ZnS ----- (argon) ------> AlFZnS (non-metallic) ;
2/3 Ti0 + 1/3 TiF3 + ZnS ----- (argon) ------> TiFZnS (metallic) ; then ,
x [AlF]2+ [ZnS]2- + (1-x) [TiF]3+ [ZnS]3- -----------> [Alx Ti1-x F](3-x)+ [ZnS](3-x)-.
These Al-Ti-ZnS composites , with the mole fraction x taken between zero and unity so that the sum (3-x) is non-integral , might be superconducting , but probably only in the low temperature range of 1020 K or so . All of the research in this area so far has demonstrated the necessity of adding two to three electrons , in an non-integral manner , to the acceptor metal's p orbitals in order to obtain a reasonably high Tc superconductor . Although this wouldn't be possible with the Al-Ti-ZnS composites , they would nevertheless be very interesting metallic materials to synthesize and study .
The theme of layered compounds as the basis of new metallic solids (and possibly of superconductors) continues in the related web page ,Exploring Some New Chemistry of Layered Compounds.
References , Notes , and Further Reading
An excellent overview of the Transition metal pnictide layer compounds is now available on the ArXiv.org website : K. Ishida , Y. Nakai , and H. Hosono , To What Extent Iron-Pnictide New Superconductors Have Been Clarified : A Progress Report (PDF , 850 KB) . This is a review of the solid state physics of these materials , with little mention of their chemical aspects , so I hope my present report will be a useful supplement in that regard to their survey .
litharge/anti-litharge structure : A.F. Wells , Structural Inorganic Chemistry , 3rd edition , Clarendon Press , Oxford (UK) , 1962 ; Fig. 160 , p. 477 .
M3D model : the crystal structure illustrations throughout my ebooks and web pages were created using the Molecules 3D Pro program from the now defunct software firm Molecular Arts Corporation , Anaheim , California . Molecules 3D was primarily designed for constructing ball-and-stick and space-fill molecular structures , but I have found it very useful for making models of nonmolecular crystal structures , too . After building the desired structure , it can be whirled and twirled about (on the computer screen) , and rocked back and forth . The following picture shows what the litharge structure looks like before it's transferred back to the main window for conversion into a static graphics image (a Windows metafile , then a JPEG or GIF image) :
These twirling and rocking features are very helpful , as they enable movement of the structure in an apparent three dimensional manner (hence the M3D name) , thus permitting a view of the remarkable and often beautiful atomic architecture of the extended atomic lattices in crystals .
Several crystal-designing software programs are commercially available , and they indeed produce splendid crystal models , but they have two main disadvantages for me : first , they are extremely expensive , more than my limited budget can afford ; and second , they are very sophisticated , requiring the user to have an extensive knowledge and understanding of crystallography , which I don't have (my molecular models are presented as illustrations only) . Molecules 3D was quite affordable , and can be used even by novices in chemistry , almost like a game of building blocks or "tinkertoy" (if you remember that from your childhood) . It's really a lot of fun and mentally stimulating , as it requires the user sometimes to "out-think the program" in order to construct the more challenging structures . Sadly , Molecules 3D is no longer available commercially . I hope it can be revived again , maybe even as freeware , by some dedicated and enterprising person .
A unused copy of Molecules-3D may still be available from an auction website such as Ebay .
By the way , the other sketches and non-structure illustrations were produced using the "ACD/ChemSketch" software program . A full-featured commercial version is available , and the software publisher , Advanced Chemistry Development Inc. , Toronto , Ontario , Canada , has kindly made the previous-to-commercial version available as freeware . Highly recommended !
LaOFeP : Y. Kamihara et al. , Iron-based Layered Superconductor : LaOFeP, J. Amer. Chem. Soc. 128 (31) , pp. 10012-10013 (2006) .
Kamihara et al. : Y. Kamihara , T. Watanabe , M. Hirano , and H. Hosono , Iron-Based Layered Superconductor La[O1-xFx]FeAs (x = 0.050.12) with Tc = 26 K, J. Amer. Chem. Soc. 130 (11) , pp. 3296-3297 (2008) .
strontium-doped : H.-H. Wen et al. , Superconductivity at 25 K in Hole-Doped La1-xSrxOFeAs, Europhys. Lett. 82 (1) , pp. 17009/pp.1-5 , (2008) .
composites : G. Mu et al. , Synthesis , Structural , and Transport Properties of the Hole-Doped Superconductor Pr1-xSrxOFeAs, Phys. Rev. B 79 (10) , 104501/6 pp. (2009) . Available as a preprint from the ArXiv.org website (PDF , 1046 KB) .
Chen et al. : G.F. Chen et al. , Superconductivity at 41 K and Its Competition with Spin-Density-Wave Instability in Layered CeO1-xFxFeAs, Phys. Rev. Lett. 100 (24) , pp. 247002-1 to 247002-4 (2008) .
chemical pressure : for example , see these two recent papers : S. Sanna et al. , Experimental Evidence of Chemical-Pressure-Controlled Superconductivity in Cuprates, ArXiv.org (PDF , 275 KB) ; G. Catalan et al. , Effect of Chemical Pressure on the Magnetic Transition of Multiferroic Bi1-xCaxFeO3, ArXiv.org (PDF , 151 KB) .
Hagenmuller effect : G. Demazeau , M. Pouchard , and P. Hagenmuller , Sur de Nouveaux Composés Oxygénés du Cobalt +III Dérivés de la Perovskite, J. Solid State Chem. 9 (3) , pp. 202-209 (1974) .
In 1974 a team of French researchers at the Université de Bordeaux , France , led by Professor Paul Hagenmuller , described striking electrical and magnetic effects in a series of cobalt(III) ternary compounds . These perovskites , with a distorted symmetry , had the general formula LnCoO3 , where Ln was a trivalent rare earth cation (Y3+, La3+, Lu3+, Gd3+, etc.) . Remarkable changes in the electrical and magnetic properties of the compounds were noted when varying their Ln cations . Hagenmuller and co-workers suggested that a transition in the electronic configuration of the cobalt(III) cation was causing the corresponding transitions in the electrical and magnetic properties experimentally observed , from the high spin cobalt with the larger Ln cations , to a low spin state in the Co(III) when the smaller Ln cations were used in the perovskite . The smaller Ln cations caused the supercubic cobaltoxygen "cage" about them to shrink . This in turn caused the cobalt hybrid orbitals used in the CoO bonds to shift from the larger outer octahedral AO , sp3d2 [high spin] , to the smaller tetragonal hybrid AO , dsp2 + spz [low spin ; my interpretation from VB theory] . That resulted in a different electronic configuration in the cobalt(III) , producing significant changes in the electrical and magnetic properties of the LnCoO3 perovskites .
See also : P.M. Raccah and J.B. Goodenough , First-Order Localized-Electron <> Collective-Electron Transition in LaCoO3, Phys. Rev. 155 (3) , pp. 932-943 (1967) ; V.G. Bhide , D.S. Rajoria , G.R. Rao , and C.N.R. Rao , Mössbauer Studies of the High-Spin Low-Spin Equilibria and the Localized-Collective Electron Transition in LaCoO3, Phys. Rev. B 6 (3) , pp. 1021-1032 (1972) ; W.H. Madhusudan , K. Jagannathan , P. Ganguly , and C.N.R. Rao , A Magnetic Susceptibility Study of Spin-State Transitions in Rare-Earth Trioxocobaltates(III), J. Chem. Soc. Dalton Trans. 1980 (8) , pp. 1397-1400 .
crystal ionic radius : all crystal ionic radii cited in this web page are per Shannon-Prewitt , and are consistent with the oxide radius of 1.40 Å (6-coordinate) : D.R. Lide (ed.) ,Ionic Radii in Crystals, CRC Handbook of Chemistry and Physics , 87th edition , CRC Press / Taylor & Francis , Boca Raton (FL) , 2006 .
Italian researchers : M. Tropeano et al. , Effect of Chemical Pressure on Spin Density Wave and Superconductivity in Undoped and 15% F-Doped La1-yYyFeAsO Compounds, Phys. Rev. B 79 (17) , pp. 174523/1-6 (2009) :
The chemical pressure provided by the partial substitution with this smaller ion size [Y3+] causes a lowering of the spin density wave temperature in the undoped compounds , as well as an increase in the superconducting transition temperatures in the doped ones . The 15% fluorine-doped samples reach a maximum critical temperature of 40.2 K for the 50% yttrium substitution.
The thesis of this present report is that a NIOS condition via fluoride or cation doping in the electron-acceptor (covalent) layers suppresses the SDW , and that the Hagenmuller effect assists in raising Tc by shrinking the crystal lattice , which in turn permits a stronger magnetic coupling of the antiparallel "extra" electrons above EF into Cooper pairs .
transient existence : M. Hoch and H.L. Johnston , Formation , Stability and Crystal Structure of the Solid Aluminum Suboxides : Al2O and AlO, J. Amer. Chem. Soc. 76 (9) , pp. 2560-2561 (1954) ; C.N. Cochran , Aluminum Suboxide Formed in Reaction of Aluminum with Alumina, J. Amer. Chem. Soc. 77 (8) , pp. 2190-2191 (1955) . See also T. Forland et al. , Measurements of Phase Equilibria in the Aluminum Aluminum Sulfide System, Acta. Chem. Scand. , Series A28 (2) , pp. 226-228 (1974) . The compound AlS has a narrow window of stability between 1010 ºC and its m.p. of 1060 ºC .
electrical conductor : M.D. Banus , T.B. Reed , and A.J. Strauss , Electrical and Magnetic Properties of TiO and VO, Phys. Rev. B 5 (8) , pp. 2775-2784 (1972) . TiO has an ambient electrical conductivity of about 333 ohm-1cm-1, and becomes superconducting at Tc ~ 0.41.0 K .
fluoride doping : S.R. Ovshinsky et al. , Superconductivity at 155 K, Phys. Rev. Lett. 58 (24) , pp. 2579-2581 (1987) ; M. Xian-Ren et al. , Zero Resistance at 148.5 K in Fluorine Implanted Y-Ba-Cu-O Compound, Solid State Commun. 64 (3) , pp. 325-326 (1987) ; K. Fukushima , H. Kurayasu , T. Tanaka , and S. Watanabe , The Influence of Fluorine Addition on the Crystal Structure and Electrical Properties of a YBa2Cu3O7-d Oxide Superconductor, Jpn. J. Appl. Phys. 28 (9) , pp. L1533-L1536 (1989) ; C.H. Chen et al. , Superlattice Modulation and Superconductivity in the Electron-Doped Nd2CuO4-xFx and Nd2-xCexCuO4 Systems, Physica C 160 (3&4) , pp. 375-380 (1989) .
hopping semiconductors : O. Berkooz , M. Malamud , and S. Shtrikman , Observation of Electron Hopping in 151Eu3S4 by Mössbauer Spectroscopy, Solid State Commun. 6 (3) , pp. 185-188 (1968) . A beautiful demonstration of a hopping semiconductor ! Eu3S4 [with Eu3+Eu2+Eu3+] is a Robin-Day Class II mixed-valent compound , but apparently lacks a suitable overlapping of its valence electron orbitals to form a true metallic bond . Its 4f7 valence electron (formally on the Eu2+) can hop over the sulfide anion to a neighbouring Eu3+ cation , but no further . This hopping is a thermally-dependent process , so that when Eu3S4 is cooled , the 4f7 electrons gradually become localized ("pinned") on their parent Eu2+ kernels , which is clearly shown by Mössbauer spectroscopy .
all-fluoride ferropnictide : P. Cheng et al. , High-Tc Superconductivity Induced by Doping Rare-Earth Elements Into CaFeAsF, Europhys. Lett. 85 (6) , pp. 67003/pp.1-4 , (2009) . Available as a preprint from the ArXiv.org website (PDF , 402 KB) .
LaONiP : T. Watanabe et al. , Nickel-Based Oxyphosphide Superconductor with a Layered Crystal Structure , LaNiOP, Inorg. Chem. 46 (19) , pp. 7719-7721 (2007) ; M. Tegel , D. Bichler , and D. Johrendt , Synthesis , Crystal Structure and Superconductivity of LaNiPO, Solid State Sci. 10 (2) , pp. 193-197 (2008) . The latter authors measured Tc = 4.3 K for LaONiP . Note the three different ways the chemical formula of the compound was written ! Of course I prefer LaONiP , since this represents its "real" layered structure , [LaO]ionic1+ [NiP]covalent1-.
recently reported : X. Zhu et al. ,Transition of Stoichiometric Sr2VO3FeAs to a Superconducting State at 37.2 K, Phys. Rev. B 79 (22) , 220512(R)/4 pp. (2009) .
KCP : K. Krogmann , Planar Complexes Containing Metal-Metal Bonds, Angew. Chem. Internat. Ed. Engl. 8 (1) , pp. 35-42 (1969) ; A.J. Epstein and J.S. Miller , Linear Chain Conductors, Scientific American 241 (4) , pp. 52-61 (October , 1979 ; a colour photograph of K1.75Pt(CN)4.1.5 H2O is on p. 54) ; J.A. Abys et al. , pp. 1-5 in Ch. 1 , Electrically Conducting Solids, Inorg. Synth. 19 , pp. 1-58 , D.F. Shriver (ed.) , John Wiley , New York (1979) ; S.T. Matsuo , J.S. Miller , E. Gebert , and A.H. Reis , Jr. , One-Dimensional K2Pt(CN)4Br0.3 . 3 H2O , A Structure Containing Five Different Types of Bonding, J. Chem. Educ. 59 (5) , pp. 361-362 (1982) ; J.M. Williams and A.J. Schultz , One-Dimensional Partially Oxidized Tetracyanoplatinate Metals : New Results and Summary, pp. 337-368 in Molecular Metals , W.E. Hatfield (ed.) , Plenum Press , New York , 1979 ; J.S. Miller and A.J. Epstein , One Dimensional Inorganic Complexes, Prog. Inorg. Chem. 20 , pp. 1-151 , S.J. Lippard (ed.) , John Wiley , New York , 1976 ; H.R. Zeller and A. Beck , Anisotropy of the Electrical Conductivity in the One-Dimensional Conductor K2[Pt(CN)4] Br0.30 . 3 (H2O), J. Phys. Chem. Solids 35 (1) , pp. 77-80 (1974) .
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 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 .
ferrimagnetic : A. Tressaud and J.M. Dance , Ferrimagnetic Fluorides, Adv. Inorg. Chem. Radiochem. 20 , H.J. Emeléus and A.G. Sharpe (eds.) , Academic Press , New York , 1977 ; pp. 133-188 .
Spectrochemical Series : A.F. Holleman , E. Wiberg , and N. Wiberg , Inorganic Chemistry , 1st edition (Engl.) , Academic Press , San Diego (CA) , 2001 ; pp. 1183-1185 . Halide anions are at the "weak end" of the Series . See also F. Basolo and R.C. Johnson , Coordination Chemistry , The Chemistry of Metal Complexes , W.A. Benjamin , New York , 1964 . The Spectrochemical Series of ligands is listed on p. 46 . Crystal field splitting of d orbital energy levels is discussed on pp. 43-49 . Fig. 2-7 , p. 33 , compares high spin [CoF6]3- with low spin [Co(NH3)6]3+. Fig. 2-13 , p. 45 , compares high spin [Fe(H2O)4]3+ with low spin [Fe(CN)6]3-.
several examples : C.P. Poole , Jr. , T. Datta , and H.A. Farach , Copper Oxide Superconductors , John Wiley , New York , 1988 ; Table VII-2 , p. 125 .
CaFFeAs with cobalt : S. Matsuishi et al. , Superconductivity Induced by Co-Doping in Quaternary Fluoroarsenide CaFeAsF, J. Amer. Chem. Soc. 130 (44) , pp. 14428-14429 (2008) .
very close to the 4s and 4p AOs : 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 .
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) .
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