Liquid structure under extreme conditions: high-pressure x-ray diffraction studies

Abstract

Under extreme conditions of high pressure and temperature, liquids can undergo substantial structural transformations as their atoms rearrange to minimise energy within a more confined volume. Understanding the structural response of liquids under extreme conditions is important across a variety of disciplines, from fundamental physics and exotic chemistry to materials and planetary science. In situ experiments and atomistic simulations can provide crucial insight into the nature of liquid-liquid phase transitions and the complex phase diagrams and melting relations of high-pressure materials. Structural changes in natural magmas at the high-pressures experienced in deep planetary interiors can have a profound impact on their physical properties, knowledge of which is important to inform geochemical models of magmatic processes. Generating the extreme conditions required to melt samples at high-pressure, whilst simultaneously measuring their liquid structure, is a considerable challenge. The measurement, analysis, and interpretation of structural data is further complicated by the inherent disordered nature of liquids at the atomic-scale. However, recent advances in high-pressure technology mean that liquid diffraction measurements are becoming more routinely feasible at synchrotron facilities around the world. This topical review examines methods for high pressure synchrotron x-ray diffraction of liquids and the wide variety of systems which have been studied by them, from simple liquid metals and their remarkable complex behaviour at high-pressure, to molecular-polymeric liquid-liquid transitions in pnicogen and chalcogen liquids, and density-driven structural transformations in water and silicate melts.

Boron Incorporation in Silicate Melt: Pressure-induced Coordination Changes and Implications for B Isotope Fractionation

“…Compression of Di-An melt results in progressive increases in coordination of all cations, as opposed to abrupt changes in coordination observed in crystalline silicate minerals. For all cations, changes in overall coordination number arise from the progressive increase in fraction of higher coordinated species at the expense of lower coordination species, consistent with previous experimental (Sanloup et al, 2013a;Sanloup et al, 2013b;Wang et al, 2014;Drewitt et al, 2015;Drewitt, 2021) and simulation (Stixrude and Karki, 2005;Adjaoud et al, 2008;de Koker et al, 2008;Spera et al, 2009;de Koker 2010;, 2015) directly compared classical MD, using the same aspherical ion model potential as Adjaoud et al (2008), and ab initio MD calculations, the latter of which show a similar development in Si-O coordination to that noted here and in ab initio MD simulations of other Ca-bearing aluminosilicate liquids (Bajgain et al, 2015b). Adjaoud et al (2008) did note concomitant increases in Mg coordination in their simulations, from ~5 at room pressure to ~7 at 24 GPa, which are broadly comparable to Mg coordination changes up to 8 GPa in this study.…”

LiquidDiffract: software for liquid total scattering analysis

“…The interference function, i(Q), tends to zero at Q = ∞ , so the effect of truncating at very high-Q ( ≳ 30Å −1 ) is often negligible. However, the effect is much more significant for high-pressure studies due to the limited accessible scattering angle when working with high-pressure instruments (Drewitt 2021). Furthermore, truncating at a lower Q max corresponds to a broadening of the sinc function, and these broader oscillations are often difficult to distinguish from real structural information (Lorch 1969;Shen et al 2004).…”

“…The study of liquid and glass structure provides insight into the behaviour and physical properties of amorphous materials that is important in many fields of research. At extreme conditions of high temperature (T) and pressure (p) an understanding of liquid structure and polyamorphic (liquid-liquid) phase transitions has important industrial applications, including novel phase discovery, and modelling of industrial melt processes (Drewitt 2021). Fluids and melts are also key to our understanding of the dynamic Earth, occurring from the crust to the core and controlling mass transfer in the deep interior, and geothermal, volcanic, and ore-forming processes at the near surface.…”

“…There is a clear need for software such as LiquidDiffract, which provides an open-source and libre tool for x-ray total scattering analysis of liquids and disordered solids. The procedures implemented to extract bulk liquid number density will be of particular benefit to researchers performing experiments at extreme conditions, as the equations of state of many liquids and glasses at high-p are poorly known (Drewitt 2021). The efficacy of LiquidDiffract to produce accurate structure factors and equation of state data of liquids under extreme conditions has been successfully demonstrated in published studies of liquid gallium to 25 GPa (Drewitt et al 2020), and liquid tantalum shockreleased from several hundred GPa (Katagiri et al 2021).…”

How to predict the density of liquids up to the Gigapascal range knowing only data obtained at ambient pressure: the Tait equation meets the phonon theory of liquids

“…Note that the bulk modulus is recently considered one of the most promising thermodynamic characteristics of lowcompressible media since its relative change can be treated as a small parameter 21,22 . In addition, this quantity directly relates to the behaviour of the liquid's structure factor 23 , entropy and isochoric heat capacity 24 , and the Grüneisen parameter 25 treated at extreme pressures by analogy to the solid state physics. Simultaneously, this analogy at saturation conditions and in the supercritical region led to the development of the phonon theory of liquid thermodynamics [26][27][28] .…”