Signature of a polyamorphic transition in the THz spectrum of vitreous GeO2

TL;DR

This study investigates the THz spectrum of vitreous GeO2 under high pressure, revealing a polyamorphic transition characterized by spectral shape changes and phonon dispersion evolution from quartz-like to rutile-like coordination.

Contribution

It provides the first inelastic x-ray scattering evidence of a polyamorphic transition in vitreous GeO2 and links spectral changes to structural coordination shifts.

Findings

Spectral shape differences across the transition in the 30-80 meV range

Cusp-like pressure dependence of elastic response

Phonon dispersion evolution from quartz-like to rutile-like coordination

Abstract

The THz spectrum of density fluctuations, $S(Q, \omega)$, of vitreous GeO$_2$ at ambient temperature was measured by inelastic x-ray scattering from ambient pressure up to pressures well beyond that of the known $\alpha$-quartz to rutile polyamorphic (PA) transition. We observe significant differences in the spectral shape measured below and above the PA transition, in particular, in the 30-80 meV range. Guided by first-principle lattice dynamics calculations, we interpret the changes in the phonon dispersion as the evolution from a quartz-like to a rutile-like coordination. Notably, such a crossover is accompanied by a cusp-like behavior in the pressure dependence of the elastic response of the system. Overall, the presented results highlight the complex fingerprint of PA phenomena on the high-frequency phonon dispersion.