Identical sites within the unit cell are not occupied

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Unformatted text preview: ructure because all crystallographically vacant for each 3Mg" replaced by 2A1"'. identical sites within the unit cell are not occupied by the same cation. A related type of defect structure occurs in valency disordered Sodium-p-alumina and related phases (57) spinels where, for example, the divalent A" cations in AB204 are replaced by equal Sodium-B-alumina has assumed tremendous numbers of M' and M"' of appropriate size. importance as a solid-state electrolyte since its Thus, in spinel itself, which can be written very high electrical conductivity was discovered MgsA116032, the 8Mg" (72 pm) can be replaced at the Ford Motor Company by J. T. Kummer by 4Li' (76pm) and 4A1"' (53pm) to give Li4and N. Weber in 1967. The compound, which has Al20O32, i.e. LiA1508. This has a defect spinel the idealized formula NaAll 1017(Na20.lA1203) 1 structure in which two-fifths of the A1 occupy was originally thought to be a form of A1203 all the tetrahedral sites: (A1~l)t[Li'A1~l]oO~. and hence called p-alumina (19 16); the presence Other compounds having this cation-disordered of Na, which was at first either undetected spinel structure are LiGa508 and LiFe5Os. or ignored, is now known to be essential for Disordering on the tetrahedral sites occurs stability. X-ray analysis shows that the structure in CuAl5S8, CuInsSs, AgA15S8 and AgInsSs, is closely related to that of spinel, no fewer i.e. (CU'A~"'),[A~~' etc. Valency disordering loss, than 50 of the 58 atoms in the unit cell being can also be achieved by replacing A" completely arranged exactly as in spinel. The large Na atoms by MI, thus necessitating replacement of half are situated exclusively in loosely packed planes the B"' by MI", e.g. (Li'),[A1"lTi'V],O~. Even together with an equal number of 0 atoms as more extensive substitution of cations has shown in Fig. 7.15; these planes are 1123pm been achieved in many cubic spinel phases, apart, being separated by the "spinel blocks". e.g. Li:Zn~1Al:1'Ge~v036 (and the Ga"' and Fe"' The close-packed oxygen layers above and below analogues), and the possibilities are virtually the Na planes are mirror images of each other limitless. 476pm apart and they are bound together not The sensitive dependence of the electrical and only by the Na atoms but by an equal number magnetic properties of spinel-type compounds of AI-0-A1 bonds. There are several other sites in the mirror plane which can physically on composition, temperature, and detailed cation accommodate Na and this permits rapid twoarrangement has proved a powerful incentive dimensional diffusion of Na within the basal for the extensive study of these compounds in connection with the solid-state electronics industry. Perhaps the best-known examples are the ferrites, including the extraordinary J. T. KUMMER,rog. Solid State Chem. 7 , 141-75 (1972). P compound magnetite Fe304 (p. 1080) which has J. H. KENNEDY, opics in Applied Physics 21, 105-41 T an inverse s...
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This note was uploaded on 10/12/2012 for the course CHEMISTRY CHM1040 taught by Professor Jhon during the Spring '12 term at Florida A&M.

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