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Abstract:
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Molecular dynamic (MD) simulations, both
classical and ab initio, of amorphous GeO2 (germania),
Al2O3 (alumina), and CdTeO compounds are presented.
We focus our attention on the structural and vibrational
properties, giving an atomic description of the short- and
intermediate-range order. Amorphous germanium oxide
under pressure was studied by means of classical MD
simulations. At normal density, the analysis of the interatomic
distances reveals that in the amorphous state there is
a short-range order dominated by a slightly distorted
(GeO4) tetrahedron. Beyond that, there is an intermediaterange
order composed by vertex-sharing tetrahedra. As
density increases, there is a structural transformation, from
a short-range order defined by the basic tetrahedron to a
basic octahedron. Consistent with this picture, the vibrational
density of states also presents big changes, where the
low frequency band shrinks, and the high frequency
becomes wider and flatter. In the case of alumina, both
classical and first principles MD calculations of amorphous
Al2O3 are reported, comparing both methodologies. Interatomic
correlations allow us to conclude that the shortrange
order is mainly composed by AlO4 tetrahedra, but in
contrast to classical MD results, also an important number
of AlO5 unit is present. The vibrational density of states
presents two main bands, a low frequency one related to
the inter-tetrahedron vibration and a high frequency band
related to the intra-tetrahedron vibration. Finally, we
present an ab initio MD calculation for the complex ternary
material CdTeO3. According our calculations, the shortrange
order of this compound consists of a number of basic
building blocks, greater than in the case of its crystalline
counterpart. The compound is characterized using pair and
angular distribution functions, coordination numbers, and a
description of the molecular units of the compound.
For example, Cd is coordinated by either six or five atoms.
In the case of Te, the chemical unit is TeO3. The most
frequent clusters are CdO6, CdO5, TeO3, and TeO4. |