Luigi Giacomazzi, Alfredo Pasquarello
Using a density-functional scheme, we study the structural and vibrational properties of vitreous germanium diselenide (v-GeSe2). Through the use of classical and first-principles molecular-dynamics methods, we generate a set of structural models showing a varying degree of chemical disorder. In particular, two types of structural concepts are represented: one in which the tetrahedral order is preserved to a very large extent, and one which reproduces the high degree of disorder in first-neighbor shells found in first-principles molecular-dynamics simulations of the liquid. The investigated structural properties include the angular distributions, the atomic arrangements in the first-neighbor shells, and the pair-correlation functions. In reciprocal space, we have calculated the x-ray and neutron total structure factors and the partial structure factors. Comparison with experiment gives overall good agreement for the models of either structural conception. We then investigate the vibrational properties via the vibrational density of states and the inelastic neutron spectrum. The considered models yield similar spectra and agree with experimental data. We also obtain infrared and Raman spectra through a density-functional scheme based on the application of finite electric fields. For these spectra, significant differences appear among the models. The comparison with experiment favors a model showing a high degree of chemical order. The Raman intensity is analyzed in terms of the underlying atomic vibrations. The assignment of the Raman companion line to Se motions in edge-sharing tetrahedra is supported.
2007
