Liquid–liquid transition and critical point in sulfur

Abstract

A liquid-liquid transition (LLT) is a peculiar phenomenon in which a singlecomponent liquid transforms into another one via a first-order phase transition. Due to their counterintuitive nature, LLTs have intrigued scientists for several years and changed our perception of the liquid state. Such LLTs have been predicted from computer simulations of water 1,2 , silicon 3 , carbon dioxide 4 , carbon 5 , hydrogen 6 and nitrogen 7. Experimental evidence has been mostly found in supercooled, i.e. metastable, liquids such as Y2O3-Al2O3 mixtures 8 , water 9 and other molecular liquids 10,11,12 , but the LLT in supercooled liquids often occurs simultaneously with crystallization, making it hard to separate the two phenomena 13. A liquid-liquid critical point (LLCP), similar to the gas-liquid critical point, has been predicted at the end of the LLT line in some cases, but so far never experimentally observed for any materials. This putative LLCP has for instance been invoked in the case of water to understand its thermodynamic anomalies 1. Here we report combined in-situ density measurements, x-ray diffraction and Raman scattering that provide direct evidence for a first-order liquid-liquid transition and a liquid-liquid critical point in sulfur. The transformation manifests itself by a sharp density jump between the low

Modeling Fluid Polyamorphism Through a Maximum-Valence Approach

“…Correspondingly, the structure factor shows a shift in the first peak to a larger wavenumber q, while the second peak changes due to polymerization. This change is similar to what was observed in a recent experiment on sulfur [6]. In addition, S(q) shows a dramatic increase as q → 0 for the points corresponding to the equilibrium between two liquid phases (see Fig.…”

“…In (a), the sharp peak, around r = 1 (in units of σ), corresponds to the length of the covalent bond, which increases upon increasing density. Simultaneously, in (b), the maximum of the structure factor (the first peak) shifts to larger wavenumbers upon increasing density, while the second peak acquires a characteristic bump, similar to what was recently observed in sulfur [6]. The divergence of the structure factor at q = 0 indicates the divergence of the isothermal compressibility in the vicinity of the LLCP.…”

“…Contrarily, more recent ab initio molecular dynamics simulations predicted a continuously decreasing average polymer chain length upon heating at 10 GPa, not associated with a change in density [5]. Recently, it was discovered that high-density sulfur, well above the liquid-gas critical pressure (in the range from 0.5 to 2.0 GPa), exhibits a LLPT indicated by a discontinuity in density from a low-density liquid (LDL) to a high-density liquid (HDL) [6]. This LLPT in polymerizing substances has also been observed in phosphorous [7] and liquid carbon [8], while being proposed to exist in selenium and tellurium [4,5].…”

Density functional and classical simulations of liquid and glassy selenium

“…In spite of these structural regularities and the miscibility for all concentrations of S-Se, S-Te (with a gap [12,13]), and Se-Te [14,15], the disordered phases of S, Se, and Te differ greatly, as emphasized in a recent review [16]. It has been known for over a century [17] that the viscosity of liquid S increases by several orders of magnitude near 160 • C. This is usually attributed to the ring-opening polymerization of S 8 units [18], and very recent combined density, x-ray diffraction, and Raman scattering measurements in sulfur indicate * r.jones@fz-juelich.de the presence of a liquid-liquid first-order phase transition and critical point at pressures up to 3 GPa [19]. Eisenberg and Tobolsky [20] proposed that a ring-chain transition also occurred in Se at 356 K, i.e., below the melting point (494 K).…”

Термодинамические Свойства Жидкого Цезия: Поиск Аномалий

“…Проблема полиморфизма веществ в жидком состоянии сохраняет актуальность, что нашло отражение, например, в новом обстоятельном обзоре на эту тему [44], где представлены итоги экспериментальных и теоретических исследований фазовых переходов в однокомпонентных жидких и аморфных средах, в основном на уровне 2019 г. В статье [44] проанализированы новые уникальные опытные данные по фазовой T −p-диаграмме серы [45]. Диаграмма включает линию ФП-2 и ее пересечение с линией равновесия жидкой и твердой фаз.…”