Vibrational properties of vitreous germania by inelastic cold neutron scattering

Abstract

Inelastic cold neutron scattering experiments have been carried out on polycrystalline (hexagonal) and vitreous GeO₂. Coherence effects are observed for both materials for ω [graphic omitted] 300 cm^–1 and it has been possible to estimate the approximate form of two average acoustic branches of the spectrum for the polycrystal. For the glass the coherence effects may be interpreted in terms of wave vector conservation conditions operating in a similar manner to that in polycrystals. The approximate shape of two average acoustic branches has been estimated together with the frequency of a further branch near the equivalent of the Brillouin zone boundary. The data have also been examined on the basis of the incoherent approximation to determine the main features of the frequency distribution G(ω) for the glass at ω < 600 cm^–1, in which region the most prominent peak is near 300 cm^–1. The low frequency peaks in G(ω) account extremely welll for the low temperature heat capacity. The neutron scattering results have been compared with the infra-red and Raman spectra of vitreous GeO₂ showing that the relation between G(ω) and the spectra is different in each case but that the infra-red absorption coefficient probably approximates reasonably well to G(ω), except below about 200 cm^–1. A comparison has also been made with a theoretical frequency distribution for a model of vitreous GeO₂. The neutron spectra for the glass show a peak at 13 cm^–1, estimated to contain about 0.05 % of the total modes and attributed to resonance modes associated with some particular, but so far unidentified, structural defects.