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

Alessandro Cunsolo,Yan Li,Ho-Kwang Mao,Daniele Antonangeli,Dima Bolmatov,Filippo Bencivenga,Roberto Verbeni,Yong Q Cai,Chaminda N Kodituwakku,Andrea Battistoni,Shibing Wang,Satoshi Tsutsui,Alfred Q R Baron

doi:10.1038/srep14996

Abstract

The THz spectrum of density fluctuations, S(Q, ω), of vitreous GeO2 at ambient temperature was measured by inelastic x-ray scattering from ambient pressure up to pressures well beyond that of the known α-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.

Highlights

Inelastic neutron (INS) and x-ray (IXS) scattering are two classic experimental techniques commonly used to probe atomic and lattice motions; severe technical difficulties hinder the observation of PA transitions
Water[8] and silica[9] are the two polyamorphic materials that have been most extensively investigated by inelastic spectroscopies, no signature of PA transitions has been reported in the THz spectrum in either case, mainly owing to two different reasons: in silica PA phenomena happen at pressures still prohibitively high for scattering measurements, while in water the PA transition is expected to take place in a deeply supercooled region, representing a sort of no man’s land in the thermodynamic plane[10]
Compared to SiO2, its structural analogous GeO2 has proven to be a better candidate for IXS investigations of PA phenomena due to both the larger tetrahedral cell, which shifts the onset of PA transitions to lower P’s, and the higher electronic number—and shorter x-ray absorption length—which substantially enhances the IXS signal from the small-sized sample suited for the use of Diamond Anvil Cells (DAC)[11]

Summary

Introduction

Inelastic neutron (INS) and x-ray (IXS) scattering are two classic experimental techniques commonly used to probe atomic and lattice motions; severe technical difficulties hinder the observation of PA transitions. A major one relates to the fact that modifications in the local order involving, e.g. the coordination number, usually disappear when the sample is recovered to ambient conditions (this doesn’t apply to long-range “densification” effects, which in glasses are usually permanent) This imposes in situ high-pressure experiments on very small samples for direct observation of PA transitions. We have studied the pressure-dependent spectrum of density fluctuation, S(Q, ω), of vitreous (v-)GeO2 at ambient temperature by in situ IXS measurements from ambient P up to 26 GPa (see Methods for further details) This P range has been chosen to well track the P-dependence below and above 9 GPa, pressure around which a sudden jump of the bond distance is reported and commonly ascribed to a transition from a tetrahedral to an octahedral local structure[6,12], or, in other terms, from an α-quartz-like to a rutile-like local lattice organization. Combining experimental results with first-principle density functional theory (DFT) calculations, we aim at detecting and explaining the signature of the PA crossover in the THZ spectrum of v-GeO2

Methods

Results

Discussion

Conclusion