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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)
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
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
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).
compound magnetite Fe304 (p. 1080) which has
J. H. KENNEDY, opics in Applied Physics 21, 105-41
an inverse s...
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