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same type of local bonding. Third, the calculation of the
mode-Grüneisen parameter shows two negative values,
pointing to the fact that
-Al2O3 is amorphouslike. Fourth,
the calculated vibrational DOS show broadened features
characteristic of an amorphous solid. Lastly, the comparison
between the calculated and measured XANES spectra sup-
ports this view. The individual XANES/ELNES spectra of
atoms with the same type of bonding environment also show
variations yet the sum of these spectra show agreement with
experiments. Although there are some minor discrepancies in
the comparison related to the relative peak strengths, they are
explained by the experimental samples which may not cor-
respond exactly to the pure stoichiometric
-Al2O3. Indeed,
molecular dynamic simulations41
on amorphous Al2O3 indi-
cated that the structure of amorphous Al2O3 has a close re-
semblance to
-Al2O3 surfaces with different coordinating
units including fivefold Al at different densities.
The calculation of the phonon spectrum in the present
model of
-Al2O3 is particularly challenging since there is
no internal symmetry in the model to reduce the calculation.
Furthermore, the calculation of the temperature-dependent
Gibb’s free energy G
P,T in
-Al2O3 points to the eventual
description of the various phases in transition alumina based
on a full thermodynamic treatment. The calculation of bulk
elastic properties indicates that
-Al2O3 is less rigid than
-Al2O3. The model used here does not include any porosity
at all. If the porous nature of
-Al2O3 is taken into account,
the mechanical properties will likely be even weaker. Such
studies will be important for future large scale modeling of

-Al2O3.