Computer simulation study of amorphous compounds: Structural and vibrational properties

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 intermediate-range 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 short-range 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 CdTeO ₃. According our calculations, the short-range 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.