M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

7 4 H I G H L I G H T S 2 0 2 3 I

X-rays show atoms move faster with increasing density by compression in a metallic glass

In-situ, high-pressure, high-energy X-ray photon correlation spectroscopy was used to study the atomic relaxation dynamics of glasses. The atomic relaxation time of a cerium-based metallic glass does not monotonically scale with overall density as typically expected, but instead is closely associated with detailed changes in atomic structures and could even show acceleration under compression.

Glasses are ubiquitous in our daily life, with a wide range of applications. In principle, every substance can form glass if its melt is cooled fast enough through the glass transition with crystallisation bypassed. Below the glass transition temperature, glasses fall out of equilibrium and behave like solids due to their slow relaxation dynamics with respect to experimental timescales. However, atomic relaxation into equilibrium states naturally and inevitably occurs all the time, representing a most prominent common feature of various glasses and considerably affecting their properties. Therefore, understanding atomic relaxation is crucial not only for fundamental research but also for applications of glasses. It is generally believed that structure determines the properties of materials. Unfortunately, conventional experimental studies on the dynamics of glass relaxation mainly relied on observing the macro-relaxation behaviour of a given physical property, making it difficult to directly correlate them with atomic structures. Moreover, the changes in the atomic structure of glass observed in conventional experiments are often subtle, which results in significant challenges in exploring and establishing the relationship between relaxation and the atomic structures of glass.

Recently, technical improvements in X-ray photon correlation spectroscopy (XPCS) by taking advantage of the coherence of synchrotron X-rays have enabled measurements of collective atomic motions of glassy samples with a high resolution in reciprocal space and broad coverage in the timescale, which provides a new microscopic approach for studying the atomic relaxation behaviour of glasses. Combining high pressure with XPCS offers a powerful tool to study atomic relaxation dynamics in glasses with tuneable structures over a wide range [1], as shown in Figure 55.

Metallic glasses (MGs) are a class of novel metallic materials with superior properties and are regarded as a model system of simple atomic glasses. Pressure can significantly tune the density and atomic structure, and even provoke pressure-induced polyamorphic phase transitions in MGs, but without thermal energy changes [2,3,4], therefore offering valuable opportunities to address the relationship between glassy relaxation dynamics and atomic structure.

In this work, the first in-situ, high-pressure, wide-angle XPCS experiment was successfully carried out using a symmetric diamond anvil cell (DAC) and a partially coherent X-ray beam with a high energy of 21.0 keV at beamline ID10. A polyamorphous cerium-based MG, Ce68Al10Cu20Co2, was used as the model system to investigate the glassy relaxation dynamics evolution through its pressure-induced polyamorphic transition with dramatic structural changes [5]. With a well-designed experimental protocol to minimise the diamond anvil absorption, pressure fluctuation and pressure gradient, high-quality XPCS data were obtained as a function of pressure up to 8.2 GPa (Figure 56).

With density increases, it is typically expected that atoms in glasses get more difficult to move or diffuse, slowing down their relaxation dynamics. This is consistent with

Fig. 55: Schematic illustration of the experimental setup for in-situ, high-energy, high-pressure XPCS experiments using a DAC.