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significantly from that in Ref. 26. The
structural information of this model is listed in Table I to-
gether with that of -Al2O3. It should be pointed out that due
to atomic relaxation, all atoms in the unit cell are crystallo-
graphically nonequivalent and the structure is more likely to
have a space group of P1. The unit cell of this model con-
tains 40 atoms, 16 Al
6Altet
and 10 Aloct
 and 24 O
12
O3-fold and 12 O4-fold so the cation percentage for Altet

Aloct

is 37.5%
62.5%. Likewise, the anion percentage for O3-fold
and O4-fold are both 50%. The reported experimental NMR
data indicated a smaller percentage of the Altet
.
20,21,23
How-
ever, the samples used in these studies were all derived at
high temperature with a large percentage of surface area, and
its structure can be significantly different from the bulk crys-
talline model used in the present study. Figure 1 shows the
crystal structure model of
-Al2O3 with distinguishing marks
for the different types of ions and polyhedra. For compari-
son, in the spinel lattice the tetrahedral site and octahedral
site percentages are 33.3%, and 66.7%, respectively. This
implies that there are more tetrahedral sites than octahedral
sites in the present model compared to the ideal spinel lat-