Perovskites Designed as Drude Metals and Ambient Superconductors
The concept of Drude electron materials was introduced in the preceding Chemexplore web page , A New Picture of Superconductivity : Lightning Bolt Electrons in a Crystal. To briefly summarize the new ideas brought forward in that essay :
* Electrons can be classified either as associated (with parent atomic kernels) or free (unassociated with any specific atoms) , depending on their environment .
* Electrons outside of atoms are free . Examples of such free electrons include common static electricity , lightning bolts , Tesla coil sparks , corona discharges from transformers , electrical arcing from appliances , fluorescent light bulbs , those old radio vacuum tubes , and cathode ray tubes (eg. CRT television sets and computer monitors) .
* Electrons inside matter are almost entirely associated in nature . That is , they are associated with any one of a variety of orbitals : atomic , molecular bonding , and molecular antibonding (rarely at ground state , eg. oxygen , nitric oxide) ; in localized bonding and non-bonding orbitals (the latter being lone pairs and inert pairs) ; and delocalized in lattice-wide crystal orbitals that form the metallic bondconduction band in metallic solids .
* In rare circumstances electrons inside matter may escape from their parent atomic kernels and become entirely free in the lattice interatomic void spaces as unassociated electrons . Pairs of such electrons could be the Cooper pairs in all the superconducting materials known to date .
* The free electrons in electrides may also be unassociated in nature . For example , in the inky , blue-black , dilute solutions of sodium metal in liquid ammonia the sodiums' 3s1 valence electrons are thought to have been popped off the Na atom into the void spaces between the ammonia molecules , leaving the Na1+ cation with the very stable Rare Gas (neon) electronic configuration coordinated by the ammonia lone pairs of electrons . This popping of the sodium electrons into the ammonia molecules' void spaces is energetically favourable , because sodium is a powerful reducing agent (E0ox = 2.71 V) and by losing its 3s1 valence electrons it can reduce its system energy .
* The associated electrons in the metallic bondconduction band in metallic solids were referred to as Fermi-Dirac electron gas because they are governed by Fermi-Dirac statistics , introduced in 1925 by E. Fermi and P. Dirac to describe the quantized energy distribution of the free electrons in metals . The energies of such electrons are subjected to the Fermi-Dirac distribution , which places them in a vast number of energy levels in the energy space of the crystal lattice of the metals .
* Any free electrons within matter were referred to as Drude electron gas, named in honour of the German physicist Paul Drude (1863-1906) who first proposed an electron theory of metals in 1900 . In Drude's theory the free (conduction) electrons in metals were treated as monoatomic gas molecules (like helium , for example) in rapid motion within the container walls of the atomic lattice . Their energy distribution was expected to obey Maxwell-Boltzmann statistics like conventional gas molecules ; however , this was incorrect , because the free electrons in metals are associated with atomic orbitals (which Drude was unaware of in 1900) and therefore must follow the quantum regulations of Fermi-Dirac statistics .
* The free electrons of Drude electron gas may be trapped in atomic containers that localize them , as with most electrides . They may also be delocalized and mobile in the lattice , as are the Cooper pairs in superconductors ; one metallic ceramic electride has been described to date . When Drude electron gas can freely circulate in a crystal lattice that material a Drude electron material will be both an electrical conductor and maybe even a superconductor .
* The location of the Cooper pairs in interatomic void spaces in conventional superconductors imposes a temperature limit on the stability of those electron pairs . The frontier orbitals (empty , higher energy level orbitals) over the parent atomic kernels are physically located in those same interatomic void spaces . As the superconducting material warms up , its atomic kernels vibrate more and more to the point where the Cooper pairs impact on them and fission into singlet electrons . At that point the singlet electrons become associated again with the frontier orbitals , which form the XO (crystal orbital = metallic bond = s,p conduction band) in the lattice . The material then becomes a conventional metallic solid and ordinary electrical conductor a Fermi-Dirac metal above this transition temperature .
A chemical trick was introduced in the Electrons web page to try to solve this temperature limitation problem experienced by conventional superconductors . Certain types of crystal structures have periodic void spaces in their lattices in which there are missing atoms . That is , the void spaces in them are very voluminous , capable of containing atoms (or ions) . For example , the well known double zinc blende compound Cu2HgI4 has stoichiometric numbers of such cation vacancies :
The voids can be represented by the symbol [ ] , so the formula of copper(I) tetraiodomercurate(II) can be written as Cu2Hg[ ]I4 . Based on this compound , isostructural zinc blendes can be chemically designed with similar tetrahedral voids , but which now contain pairs of electrons that have been popped off neighbouring zerovalent metal atoms . For example , the series of compounds M0[ ]Si2S4 = M2+[**]Si2S4 (M = Zn , Cd , and Hg) should have such a zinc blende crystal structure with electron pairs in the voids :
Zinc (E0ox = 0.7618 V) and cadmium (E0ox = 0.403 V) are natural reducers , so they would readily transfer their ns2 valence shell electrons into the tetrahedral voids . Mercury , however , is a Noble Metal redox-wise (E0ox = 0.851 V) and strongly resists giving up its 6s2 inert pairs of valence electrons . If the compound Hg0[ ]Si2S4 = Hg2+[**]Si2S4 is prepared under high temperaturehigh pressure (HPHT) conditions , the sulfide anions will press down on the mercury atoms and pop the 6s2 valence electrons into the voids . The underlying electrophilic Hg2+ kernels will be tetrahedrally coordinated by the nucleophilic sulfides , which are natural reducers (E0ox = 0.476 V) and will form strong coordinate covalent S>Hg bonds with them .
The M2+[**]Si2S4 compounds (M = Zn , Cd , and Hg) are , in effect , electrides in which Zn , Cd , and Hg have been dissolved in the silicon disulfide host matrix , while their ns2 valence shell electrons have been popped into adjacent atom-sized tetrahedral voids . Solution of the zerovalent metal atoms will been energetically facilitated by the formation of strong S>M coordinate covalent bonds and the relocation of the ns2 electron pairs into the voluminous tetrahedral voids . Solution of the natural reducers Zn and Cd in the SiS2 host lattice should be easy and thermodynamically favorable ; solution of the Noble Metal Hg in it would be unnatural and would require HPHT conditions in a powerful laboratory press , and would be facilitated by the formation of the strong S>Hg coordinate covalent bonds , which would stabilize Hg2+[**]Si2S4 at STP when the compound is cooled down to room temperature and depressurized .
Given the very simple , compact crystal structure and chemical composition of the zinc blendes and the high (stoichiometric) concentration of Drude electron gas in their tetrahedral voids , they were referred to as super-electrides , and considered to be a sort of high performance version of the electrides known to date . Two other types of crystal structures having voids which might similarly be used to contain electrons popped off zerovalent atoms in the lattice were briefly discussed : the rhenium trioxide structure and the layered cadmium iodide structure .
The rhenium trioxide supercubic crystal structure is particularly appealing in our search for suitable crystal containers for Drude electron gas . It's isotropic , with an identical atomic arrangement along each of its three crystal axes . It's very compact and would be an even more efficient container , in atom economy terms , than the zinc blende structure . Most importantly , it has three dimensional channels of enormous void spaces which are capable of holding even large cations such as K1+, Sr2+, and Ba2+. These void channels would be magnificent pathways in the crystal for the Drude electron gas to pour through under an applied potential difference .
The example of supercompressed bismuth(III) iodide was discussed as a possible ReO3 structure in which the Bi(III) 6s2 inert pairs have been popped into the central voids of the BiI3 lattice . Another interesting MX3 compound came to mind after the Electrons web page was posted to Chemexplore . The industrial chemical phosphorus(III) chloride , PCl3 , might similarly be supercompressed into the ReO3 structure . Phosphorus trichloride is normally a clear , colorless liquid , b.p. 76 ºC , which hydrolyzes very rapidly to phosphorous acid . Its molecules have a trigonal pyramid shape with the phosphorus 3s2 electrons in a stereochemical lone pair on the pyramid apex :
The above sketch was copied from the Wikipedia web page , Phosphorus trichloride . My thanks to the author of this graphic , and Wikipedia , for implied permission to reproduce it here .
The stereochemical directionality of the phosphorus lone pair suggests a hybridization of the valence shell 3s and 3p orbitals into the tetrahedral sp3 hybrid orbital . This , and the compression of the PCl3 molecule into the ReO3 structure , are shown in the following sketch :
From these simple little Valence Bond sketches we can predict that a low to medium level compression of PCl3 will convert it into a nonmetallic ReO3 form , while high pressure will be required to convert it into the ReO3 form and pop the phosphorus 3s2 valence electrons into the voluminous central voids in the structure . When relocated there these electron pairs will be Drude free electron gas , and this metallic form of PCl3 could possibly be superconducting .
It should be noted that in the metallic ReO3 form the phosphorus atoms are bonded by the chlorine atoms as P(V) , not as the original P(III) . This shouldn't be a problem as P(V) is a low energy species redox-wise and isn't known to be either electrophilic or an oxidizer (as illustrated by P2O5 and PCl5 , for example) . In fact , phosphorus(III) molecules are all chemically reducing in nature , and are easily oxidized to the corresponding phosphorus(V) compounds . This suggests that the P(III) kernels can readily lose the 3s2 electron pairs . Therefore the P(V) would have no inclination to try to recapture the 3s2 electrons from their voids in the metallic compound PV[**]Cl3 , which should thus be stable at STP when cooled to room temperature and depressurized .
Phosphorus(V) can be octahedrally bonded by both fluorine and chlorine atoms . The hexafluorophosphate anion , PF61- , is well known , and the PCl61- anion occurs in the crystalline , solid form of phosphorus(V) chloride , PCl5 . This compound is thought to be composed of PCl41+ cations (with tetrahedral P) and PCl61- anions (with octahedral P) : R.W. Suter et al. , Nature of Phosphorus(V) Chloride in Ionizing and Nonionizing Solvents, J. Amer. Chem. Soc. 95 (5) , pp. 1474-1479 (1973) [DOI] . Two-coordinate , linear halide anions are also well known in perovskites , so there should be no theoretical objection to the possibility of PCl3 having a high pressure ReO3 crystal structure .
Another interesting point about both the nonmetallic and metallic PCl3 forms is that in their compact ReO3 structure the octahedrally coordinated phosphorus atoms would be shielded by the chlorines from attacking nucleophiles , such as water molecules . A similar situation occurs with the sulfur hexafluoride molecule , which is chemically quite inert under most conditions , rather like the fluorocarbon compounds and PTFE polymers . Sulfur tetrafluoride , by comparison , is missing two equatorial fluorides and so is more vulnerable to nucleophilic attack ; it's a violently reactive chemical compound ! We can predict that the high pressure ReO3 forms of PCl3 would be fairly unreactive ; in particular , they would be resistant to hyrolysis under moderate reaction conditions .
As a key phosphorus reagent phosphorus trichloride is a gateway industrial precursor for a vast number of downstream chemical commodities . Wouldn't it be marvelous to create a super-duper ambient superconductor from it ? The fly in the ointment in this rosy scenario is , of course , the tremendous pressures in the super-presses that would be required for its conversion into the metallic ReO3 form . However , where there's a will , there's a way , as the old saying goes . And where there's a profit incentive , there will always be a commercial and industrial will !
Perovskites as Crystal Containers for the Drude Electron Gas
The empirical formula for the metallic ReO3 form of PCl3 , [**]PVCl3 , is reminiscent of that of the perovskites , AMX3 , where A is a large cation , M is a small cation , and X is an anion . At first glance it would seem impossible to use perovskites as the crystal container for Drude electron gas , as the voluminous A voids are completely filled with A cations . But a simple chemical trick can be used to circumvent this problem : the perovskite formula is doubled to A2M2X6 . Then , one of the A positions is reserved for the empty void space , and the second A position will be occupied by a zerovalent A atom , whose single ns1 valence shell electron or pair of ns2 electrons will be popped into the adjacent voids : A1+[*]M2X6 or A2+[**]M2X6 . The following sketch illustrates this design concept for perovskites with fluoride X anions :
The host matrix , AlF3 , already has a distorted sort of ReO3 crystal structure :
The above sketch was copied from the Wikipedia web page Aluminum fluoride . Again , my thanks to the author of this graphic , and Wikipedia , for implied permission to reproduce it here .
A ball-and-stick representation of the same structure (for gallium trifluoride) is shown below :
The above sketch was copied from the Wikipedia web page Gallium(III) fluoride . Again , my thanks to the author of this graphic , and Wikipedia , for implied permission to reproduce it here .
It should be quite easy to insert the strongly reducing Na0 , Ca0 , and Cd0 atoms into the large A voids surrounded by the AlF6 polyhedra , with the metals' respective 3s1, 4s2, and 5s2 valence electrons being popped into adjacent A voids :
Sodium , calcium , and cadmium super-electrides would thus be formed in the AlF3 lattice . Although one might think the Na0 and Ca0 would reduce the Al3+ to Al0 , in fact this probably wouldn't occur , just as Na0 doesn't reduce the ammonia molecules to amide anions and hydrogen gas when it forms the sodiumammonia electrides in liquid ammonia (although if catalyzed by iron salts or a rust impurity in the ammonia , it will) :
Na0 (m.p. 98 ºC) + 2 AlF3 (sublimes 1276 ºC) -------- [heat together in an inert atmosphere such as pure , dry nitrogen or argon] -------> Na1+[*]Al2F6 .
Since sodium metal is highly reactive and somewhat of a nuisance to handle in experimental manipulations , a simpler and easier procedure would be to use aluminum metal powder as the Drude electron gas source . Alfa-Aesar offers a grade of reagent aluminum , 40+325 mesh , 99.8% pure , which might be suitable for these electride syntheses with an AlF3 host lattice (caution : extremely finely divided metal powders can be pyrophoric !) :
NaF (m.p. 996 ºC) + 1/3 Al0 (m.p. 660 ºC) + 5/3 AlF3 -------- [heat] -------> Na1+[*]Al2F6 ;
Ca0 (m.p. 842 ºC) + 2 AlF3 -------- [heat] -------> Ca2+[**]Al2F6 ; alternately ,
CaF2 (m.p. 1418 ºC) + 2/3 Al0 + 4/3 AlF3 -------- [heat] -------> Ca2+[**]Al2F6 ;
Cd0 (m.p. 321 ºC) + 2 AlF3 -------- [heat] -------> Cd2+[**]Al2F6 ; alternately ,
CdF2 (m.p. 1075 ºC) + 2/3 Al0 + 4/3 AlF3 -------- [heat] -------> Cd2+[**]Al2F6 .
It should be noted that the two intermediate , low-valent electronic states of aluminum , Al(I) and Al(II) [3s2 and 3s1, respectively] are metastable , and then only at very high temperatures :
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) [DOI] ; C.N. Cochran , Aluminum Suboxide Formed in Reaction of Aluminum with Alumina, J. Amer. Chem. Soc. 77 (8) , pp. 2190-2191 (1955) [DOI] . 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) [PDF , 375 KB ; DJVU , 109 KB ; a suitable DjVu reader for your computer can be downloaded for free from djvu.org . The WinDjView reader v. 1.0.3 for older FAT 32 Windows OS can be downloaded for free from FileHorse] . The compound AlS has a narrow window of stability between 1010 ºC and its m.p. of 1060 ºC .
Thus , there are essentially only two stable valence states for aluminum : zerovalent , as in the metal itself , and Al(III) , in its many compounds . If the perovskites outlined above can be successfully formed when Na0 is inserted into 2 AlF3 , formation of the Na1+[*]Al2F6 super-electride should be easier and more energetically favorable than a clean reduction of the Al3+ to Al0 .
While a variety of Alkali , Alkaline Earth , and IIB/12 metal elements might undergo this super-electride formation process , Na , Ca , and Cd were chosen specifically because the perovskites they might form with AlF3 would likely have a cubic symmetry , as was calculated using the Goldschmidt equation for the perovskite tolerance factors :
Such a cubic symmetry would be desireable , if not essential , for maintaining the integrity of the large A voids in which the Drude electron gas resides . A tolerance factor of t = 0.901.00 is usually considered to be an indicator of cubic symmetry for the perovskite concerned . Outside of those limits its symmetry can distort into other crystal structures . For example , with t < 0.85 the ilmenite structure (also having the AMX3 empirical formula) may result in the crystalline solid . The tolerance factors presented in the above Table should be considered as approximate only . Lacking any data to the contrary the value of r = 1.15 Å for linear fluoride , CN = 2 , per Shannon and Prewitt , was used in the calculations (the crystal ionic radius for octahedral fluoride anion , CN = 6 , is 1.33 Å) .
As can be seen , the substitution of the somewhat larger Ga3+ (r = 0.62 Å) and In3+ (r = 0.80 Å) for the rather small Al3+ (r = 0.54 Å) as the M component of the AMX3 perovskites results in improved tolerance factors for the analogous Ga and In compounds . On a practical note , the host reagents GaF3 and InF3 are both very expensive chemicals , having been derived from the rare and costly gallium and indium . By comparison aluminum is an abundant and inexpensive element and commercial commodity . Also note that Ga3+ and In3+ can be reduced to lower-valent forms , Ga2+ and In1+, respectively . The compounds GaCl2 , InCl , InBr , and InI are well-known (and are commercially available) , but apparently Ga(II) and In(I) fluorides are unknown . In any case , the use of gallium and/or indium in these super-electride formulations is probably impractical .
Lead and tin might form super-electrides in an inorganic fluoride matrix . While actually being base metals redox-wise , they are close to being Noble Metals :
Pb0 2e¯ --------> Pb2+ ; E0ox = 0.1262 V ;
Sn0 2e¯ --------> Sn2+ ; E0ox = 0.1375 V .
The crystal ionic radius of Pb2+, CN = 12 (as in the perovskite A position) , per Shannon and Prewitt , is r = 1.49 Å . This is too large to be accommodated with AlF3 ; the tolerance factor for Pb2+AlF3 is t = 1.10 . However , if YF3 is used as the matrix for the lead compound , the larger Y3+ cation (r = 0.90 Å) can form a cubic symmetry perovskite with Pb2+ ; the tolerance factor for Pb2+YF3 is now t = 0.91 . Tin should form the analogous Sn2+YF3 :
Pb0 (m.p. 327 ºC) + 2 YF3 (m.p. 1387 ºC) -------- [heat together in an inert
atmosphere such as pure , dry nitrogen or argon] -------> Pb2+[**]Y2F6 ;
Sn0 (m.p. 232 ºC) + 2 YF3 -------- [heat] -------> Sn2+[**]Y2F6 .
Yttrium trifluoride has a very complex crystal structure , quite different from the simple ReO3 structures of AlF3 , GaF3 , and InF3 . Nevertheless , its component ions should be able to move about and rearrange to the perovskite structure during the formation of Pb2+[**]Y2F6 and Sn2+[**]Y2F6 during their high temperature syntheses (some pressure might be required as well) .
The smaller M component of the AMX3 perovskites might also be the source of the Drude electron gas . As before , one of the voluminous A spaces in A2M2X6 is set aside for the popped electrons , while the other A position is filled by the larger A cation . One of the M spaces is reserved for the zerovalent M element that will provide the Drude electrons ; its underlying cationic kernel will be much smaller than the A cation . The second M space will be utilized by a second small , chemically inert (in the redox sense) cation . The empirical formula for this latter sort of perovskite super-electride should look somewhat like this : Alarge [ ] M0small Msmall X6 .
If the X anions are fluorides the A and M cations must be relatively low-valent . We are restricted in this selection , since metal atoms in valence states 4 , 5 , 6 , and 7 will all be chemically reduced to lower valence states by the added electrons from the M0 component . This is why the redox-labile Transition Metal cations have been disregarded in the design of these hypothetical Drude electron materials . The following sketch illustrates a strategy for the synthesis of the super-electrides of lithium , magnesium , and zinc in the LaF3AlF3 matrix :
La3+ is the largest available trivalent cation (r = 1.36 Å , CN = 12) and fortunately LaF3 is a moderately priced chemical reagent and should be able to form cubic symmetry perovskites with the much smaller Li1+ (r = 0.76 Å , CN = 6) , Mg2+ (0.72 Å) , and Zn2+ (0.74 Å) cations : for La3+Li1+F3, t = 0.93 ; for La3+Mg2+F3, t = 0.95 ; and for La3+Zn2+F3, t = 0.94 . There might be some distortion around the Al3+ centers , though : for La3+Al3+F3, t = 1.05 . Again , the somewhat larger Ga3+ (La3+Ga3+F3, t = 1.00) and In3+ (La3+In3+F3, t = 0.91) would provide a better fit with La3+ than Al3+, but are economically impractical .
This latter series of super-electride fluorides might be synthesized by inserting lithium , magnesium , and zinc into the LaF3AlF3 host matrix :
Li0 (m.p. 180.5 ºC) + LaF3 (m.p. 1493 ºC) + AlF3 (sublimes 1276 ºC)
-------- [heat] -------> La [*] Li Al F6 .
Again , lithium metal would be difficult to handle in this synthesis ; as before , the more practical aluminum metal powder (medium fineness) could be used as the Drude electron gas source :
LiF (m.p. 848 ºC) + LaF3 + 1/3 Al0 (m.p. 660 ºC) + 2/3 AlF3 -------- [heat] -------> La [*] Li Al F6 ;
Mg0 (m.p. 650 ºC) + LaF3 + AlF3 -------- [heat] -------> La [**] Mg Al F6 ; alternately ,
MgF2 (m.p. 1263 ºC) + LaF3 + 2/3 Al0 + 1/3 AlF3 -------- [heat] -------> La [**] Mg Al F6 ;
Zn0 (m.p.420 ºC) + LaF3 + AlF3 -------- [heat] -------> La [**] Zn Al F6 ; alternately ,
ZnF2 (m.p. 872 ºC) + LaF3 + 2/3 Al0 + 1/3 AlF3 -------- [heat] -------> La [**] Zn Al F6 .
In the Electrons web page the possibility of forming super-electride soda-lime glass fibers was mentioned . Low melting fluoride glass fibers have also been studied and developed , the most stable of which is ZBLAN (ZrF4BaF2LaF3AlF3NaF) . ZBLAN fibers are spun from a melt of the five fluoride compounds at 310 ºC . They apparently are somewhat fragile and are sensitive to acid hydrolysis . Analogous fluoride glass fibers might similarly be developed from blends of the various types of super-electride fluorides discussed above . These could be of immense importance and value to future technology if they proved to be high temperature superconductors .
a-Alumina as the Host Matrix for Super-electrides
Alumina isn't a perovskite , but its empirical formula somewhat resembles theirs : Al2O3 = Al3+(AlO3)3-, vs. AMX3 . Its structure more closely resembles that of the ilmenites such as FeTiO3 (GIF image , 19 KB) . a-Alumina , which is the common , refractory , crystalline compound produced in the Bayer Process for conversion into aluminum metal , could be an interesting substrate for hosting super-electride Drude electron materials . Doping varying mole ratios of Na1+ into a-alumina results in the formation of the well-known ionic conductor , b-alumina :
The above sketch was copied from an excellent (and recommended) web page about beta-alumina . My thanks to the author and/or copyright holder of this graphic .
Insertion of the sodium cations , which are much larger (r = 1.02 Å , CN = 6) than the aluminum cations (r = 0.54 Å , CN = 6) into the a-alumina lattice causes the structure to split into layers ; the sodium cations are hosted in the interlayer spaces . When b-alumina is heated , the Na1+ cations are able to migrate upfield under an applied potential difference , making the material an ionic not electronic ; there are no free electrons in the structure conductor .
The gemstones ruby and sapphire consist mostly of a-alumina with small percentages of M3+ Transition Metal cations . Ruby contains from 0.04% (pale red) to 0.5% (deep red) of Cr3+, which has replaced the same number of Al3+ cations . Similarly sapphire is mainly a-alumina with a small percentage of Fe3+ and Ti3+ replacement cations . a-Alumina and ruby are discussed in some detail in W.J. Moore's excellent textbook , Seven Solid States , An Introduction to the Chemistry and Physics of Solids , W.A. Benjamin , New York , 1967 [ABE] ; Ch. 6 , Ruby, pp. 163-188 . The corundum crystal structure of a-alumina is presented in several sketches in Fig. 6.1 , p. 165 . The AlO6 octahedron in alumina is seen to be quite irregular in its bond angles and bond lengths .
WebElements offers a polyhedral representation of the a-alumina crystal structure :
My thanks to WebElements for the above sketch , which is from their web page Aluminium Oxide .
The repeating units of pairs of AlO6 octahedrons , separated by atom-sized void spaces : ...... Al2O3 [ ] Al2O3 [ ] Al2O3 [ ] ...... are clearly shown in the WebElements picture . This periodic repetition of voids in the a-alumina lattice is also illustrated in an expanded form in the following diagram of a stack of Al3+ [ ] layers :
The above sketch was copied from a brief Princeton University web page about alumina . My thanks to the author and/or copyright holder of this graphic .
The periodic repetition of the void spaces (white spheres) in the planes of Al3+ cations (black spheres) is readily apparent in this nice representation of the hexagonal alumina structure . The oxide anions (mercifully not shown for clarity) are presumably located in between the Al3+ [ ] layers .
The atom-sized void spaces in a-alumina could be targeted for insertion of the strongly reducing Alkali and Alkaline Earth element atoms (and maybe Zn and Cd as well) . The chemical trick of taking a 1 : 2 mole ratio of M0 and host substrate could be employed once again ; that is , the M0 base cations would be accomodated in the voids of Al2O3 no. 1 , while the popped ns1 valence electrons , or ns2 pairs of electrons , would be hosted in the voids of Al2O3 no. 2 :
Na0 (m.p. 98 ºC) + 2 Al2O3 (m.p. 2054 ºC) -------- [heat] -------> Na1+[*]Al4O6 ;
alternately , using aluminum metal powder as the Drude electron gas source ,
½ Na2CO3 (m.p. 856 ºC) + 1/3 Al0 (m.p. 660 ºC) + 11/6 Al2O3
-------- [heat , flowing inert atmosphere] -------> Na1+[*]Al4O6 + ½ CO2 (g) ;
Ca0 (m.p. 842 ºC) + 2 Al2O3 -------- [heat] -------> Ca2+[**]Al4O6 ; alternately ,
CaO (m.p. 1418 ºC) + 2/3 Al0 + 5/3 Al2O3 -------- [heat] -------> Ca2+[**]Al4O6 ;
Zn0 (m.p. 420 ºC) + 2 Al2O3 -------- [heat] -------> Zn2+[**]Al4O6 ; alternately ,
ZnO (m.p. 1974 ºC) + 2/3 Al0 + 5/3 Al2O3 -------- [heat] -------> Zn2+[**]Al4O6 .
The careful insertion of Na0 into a-alumina could lead to the production of a b-alumina structure at low doping levels ; at a stoichiometric 1 : 2 level the resulting compound [Na1+]Al2O3 [*]Al2O3 could conceivably have the cubic rocksalt structure . In any case , if stable super-electrides can be synthesized from such a-alumina adducts with M0 reducers , the resulting materials should be Drude electron materials and maybe even high temperature superconductors .
Al4F9 : A Fluorocorundum Drude Electron Material
Using aluminum metal as the source of the Drude electron gas in super-electrides poses an interesting challenge . With respect to its valence states , aluminum is all or nothing : the zerovalent metal atoms with a 3s2 3p1 electronic configuration , or the trivalent cation Al3+, without the valence electrons . There are no stable Al(I) and Al(II) compounds at STP , as discussed above . Methods of designing new solid state structures with singlet electrons or pairs of electrons in the empty cation void spaces so as to obtain Drude metals have been discussed in these recent web pages , but how can three valence electrons from a single donor atom be accomodated in a host structure ?
It might be possible to insert aluminum atoms into the large ReO3 type of central cavities in the versatile aluminum fluoride host lattice (sketch above) . The three aluminum valence electrons would be dispersed in additional central cavities provided for them . Al3+cations are much too small (r = 0.54 Å , CN = 6) to form an Al(AlF3) perovskite ; however , they might form a satisfactory Al2F3 corundum crystal structure , a fluorocorundum . The crystal ionic radii of the oxide and fluoride anions are fairly similar (1.40 Å and 1.33 Å respectively , CN = 6) , so an Al2F3 structure isn't unreasonable . Aluminum is a moderately strong reducing agent (E0ox = 1.662 V , 2.069 V in a fluoride environment) , which would thermodynamically drive the solution of the Al0 atoms in a stabilizing fluoride matrix .
A mole ratio of one gram atom equivalent of aluminum metal to three gram formula equivalents of AlF3 would be observed : the first equivalent of AlF3 would contain the residual Al3+cations as Al2F3 ; the second equivalent would contain two of the popped valence electrons as Al[**]F3 ; and the third equivalent would hold the third valence (singlet) electron as Al[*]F3 :
Al0 (m.p. 660 ºC) + 3 AlF3 (sublimes 1276 ºC) ----- (1) grind together ; (2) form a pellet ; (3) heat in an inert atmosphere such as pure , dry N2 or Ar -------> Al2F3Al[**]F3Al[*]F3 = Al4F9 .
The product of the initial Al0 insertion , Al2F3, should establish the crystal structure of the material as the corundum type , which will provide many cation vacancies for hosting the popped valence electrons . Al4F9 should be a fluorocorundum , a Drude electron material , and it might be a high temperature superconductor . Aluminum is one of the commonest and cheapest of all the metals used in modern society , and is produced in vast amounts worldwide . Finding its rather simple derivative Al4F9 to be a high temperature superconductor would be a major accomplishment for the solid state chemist who synthesizes and studies it .
Related web pages in this series about Drude electron materials :
A New Picture of Superconductivity : Lightning Bolt Electrons in a Crystal ;
Rocksalts Designed as Super-electrides , Drude Metals , and as Possible High Temperature Superconductors ;
Chromium as the Guest Atom in Super-electride Drude Metals ;
Lead , Tin , and Bismuth as the Guest Atoms in Super-electride Drude Metals ;
* Betaines and Electrides : From Sugar Beets and Baby Shampoo to Superconductors ; and ,
* Drude Electron Materials Having Rutile and Layered Structures .
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