Density Dependence of the Dynamics in Supercooled Liquids and Glasses—Insights from High-Pressure X-Ray Photon Correlation Spectroscopy

Glasses form during the tremendous, highly non-linear increase of the viscosity of supercooled liquids upon cooling, when the relaxation time becomes larger than the typical observation time [1]. As such, they are defined dynamically, and their properties up to the macroscopic scale depend heavily on the relaxation processes taking place at the microscale [2]. A consequence is that the complete description of a glass state and properties passes by a combinatory dynamical and structural approach. In metallic glasses, the dynamical information can often be obtained from X-Ray Photon Correlation Spectroscopy (XPCS), an experimental technic that takes advantage of the coherent nature of x-ray beams at 3^rd and 4^th generation synchrotron sources to monitor the timescale and nature of the relaxation processes at the atomic level [3].<br/>The past fifteen years have seen many successes from atomic scale XPCS, leading to a deeper understanding of the dynamical behavior of glasses across the full temperature range, from the deep glassy state at room temperature to the supercooled liquid state above the glass transition [4]. However, the effect of density on this dynamical description of the glassy/liquid state remains scarce at best, mostly due to the experimental difficulty of coupling high pressure environments to XPCS. Taking advantage of the exceptional coherence properties of 4^th generation synchrotron sources, our recent development of high-pressure XPCS (HP-XPCS) waived these limitations, and allowed for the monitoring of the internal dynamics of glasses in-situ under extreme conditions of pressure [5].<br/>In this talk, I will discuss the pressure effect on the atomic scale relaxation phenomena of a prototypical bulk metallic glass system, Pt_42.5Cu₂₇Ni_9.5P₂₁, as observed from HP-XPCS. At room temperature, a dichotomy appears between structure, which only reveals a monotonous densification, and dynamics, which initially reveals a surprising acceleration of the dynamics by a factor 30, challenging a pure free volume approach. A second step at higher pressure consists in a slow-down of the relaxation processes, following the typical physical aging observed at atmospheric pressure. In the supercooled liquid state, HP-XPCS shows that pressure changes the liquid’s fragility and shifts the glass transition temperature by 8 K/GPa, a factor twice higher than that obtained from ex-situ measurements on the recovered high pressure quenched glasses, showing the necessity of performing in-situ measurement for the determination of the liquid dynamics under pressure.<br/><br/><br/>[1] P.G. Debenedetti, F.H. Stilinger, <i>Supercooled liquids and the glass transition</i>, Nature <b>410</b>, 259-267 (2001)<br/>[2] W.H. Wang, <i>Dynamic relaxations and relaxation-property relationships in metallic glasses</i>, Progress in Materials Science <b>106</b>, 100561 (2019)<br/>[3] F. Lehmkühler, W. Roseker, G. Grübel, <i>From Femtoseconds to Hours – Measuring Dynamics over 18 Orders of Magnitude with Coherent X-rays</i>, Appl. Sci. <b>11</b>, 6179 (2021)<br/>[4] B. Ruta, E. Pineda, Z. Evenson, <i>Relaxation processes and physical aging in metallic glasses</i>, J. Phys. Condens. Matter. <b>29</b>, 503002 (2017)<br/>[5] A. Cornet et al., <i>High-pressure X-Ray photon correlation spectroscopy at fourth-generation synchrotron sources</i>, J. Synchrotron Rad. 31, 527-539 (2024)<br/>[6] A. Cornet et al., <i>Denser glasses relax faster: Enhanced atomic mobility and anomalous particle displacement under in-situ high pressure compression of metallic glasses</i>, Acta Materialia 255, 119065 (2023)