Deciphering the intrinsic dynamics from the beam-induced atomic motions in oxide glasses.

Yuriy Chushkin

doi:10.1107/s1600577520009753

Abstract

Probing the microscopic slow structural relaxation in oxide glasses by X-ray photon correlation spectroscopy (XPCS) revealed faster than expected dynamics induced by the X-ray illumination. The fast beam-induced dynamics mask true slow structural relaxation in glasses and challenges application of XPCS to probe the atomic dynamics in oxide glasses. Here an approach that allows estimation of the true relaxation time of the sample in the presence of beam-induced dynamics is presented. The method requires two measurements either with different X-ray beam intensities or at different temperatures. Using numerical simulations it is shown that the slowest estimated true relaxation time is limited by the accuracy of the measured relaxation times of the sample. By analyzing the reported microscopic dynamics in SiO2, GeO2 and B2O3 glasses, itis concluded that the beam-induced dynamics show rich behavior depending on the sample.

Highlights

Upon rapid cooling of a liquid below its melting temperature the liquid may enter into a supercooled liquid metastable state by avoiding crystallization (Doremus, 1994; Varshneya & Mauro, 2019)
The measurements of microscopic dynamics in oxide glasses by X-ray photon correlation spectroscopy (XPCS) are inevitably affected by the beam-induced structural rearrangements that preclude determination of the true sample structural relaxation time at the atomic length scale
When beam-induced atomic motion is a linear function of the X-ray flux, by performing two measurements, either at two different temperatures or with two different fluxes, it is possible to estimate the true sample relaxation time in simple glasses up to a certain extent into the glassy state

Summary

Introduction

Upon rapid cooling of a liquid below its melting temperature the liquid may enter into a supercooled liquid metastable state by avoiding crystallization (Doremus, 1994; Varshneya & Mauro, 2019). Further cooling increases viscosity and transforms the liquid into a glass at the glass transition temperature Tg. Glasses are amorphous solids and their structural relaxation is too slow to be observed at the laboratory time scale. XPCS applied to metallic glasses revealed the dynamical transition at Tg when the stretched exponential decay in the supercooled liquid state changes to a compressed exponential behavior in the glass (Ruta et al, 2012). Recent investigations of microscopic dynamics in oxide glasses by XPCS revealed faster than expected relaxation times in a deep glassy state related to the beam-induced effect (Ruta et al, 2017; Dallari et al, 2019; Pintori et al, 2019; Holzweber et al, 2019). The results show that the slowest possible estimated time depends on the accuracy of the measurements

Mathematical analysis

Numerical simulations

Discussion

Conclusions