Polyamorphism and liquid–liquid phase transitions: challenges for experiment and theory

Abstract

Phase transitions in the liquid state can be related to pressure-driven fluctuations developed in the density (i.e., the inverse of the molar volume; ρ = 1/V) or the entropy (S(T)) rather than by gradients in the chemical potential (μ(X), where X is the chemical composition). Experiments and liquid simulation studies now show that such transitions are likely to exist within systems with a wide range of chemical bonding types. The observations permit us to complete the trilogy of expected liquid state responses to changes in P and T as well as μ(X), as is the case among crystalline solids. Large structure-property changes occurring within non-ergodic amorphous solids as a function of P and T are also observed, that are generally termed 'polyamorphism'. The polyamorphic changes can map on to underlying density- or entropy-driven L-L transitions. Studying these phenomena poses challenges to experimental studies and liquid simulations. Experiments must be carried out over a wide P-T range for in situ structure-property determinations, often in a highly metastable regime. It is expected that L-L transitions often occur below the melting line, so that studies encounter competing crystallization phenomena. Simulation studies of liquid state polyamorphism must involve large system sizes, and examine system behaviour at low T into the deeply supercooled regime, with distance and timescales long enough to sample characteristic density/entropy fluctuations. These conditions must be achieved for systems with different bonding environments, that can change abruptly across the polyamorphic transitions. Here we discuss opportunities for future work using simulations combined with neutron and x-ray amorphous scattering techniques, with special reference to the behaviour of two polyamorphic systems: amorphous Si and supercooled YO-AlO liquids.

Structure of molten titanium dioxide

“…In a single oxide liquid, no such local compositional fluctuations are expected, but local density fluctuations may still occur, and these are associated not with other possible stoichiometries, but with other possible polymorphs which are sampled locally as regions of the melt explore the underlying energy landscape. This is qualitatively similar to the concept of polymorphoids expounded by Minaev et al [83], and related phenomenology is important in the concepts of polyamorphism and liquid-liquid phase transitions [84].…”

Anomalous and normal dependence of the sound velocity in the liquid Bi–Sb system

“…The physical properties, such as density, heat capacity and sound velocity of some liquid elements have been found to vary in a complex manner across pressuretemperature phase space [1]. Observed behaviors have included non-monotonous temperature and pressure dependences as well as discontinuities which have been taken to indicate the possible existence of liquid-liquid transitions.…”

Microscopic identification of the order parameter governing liquid–liquid transition in a molecular liquid

“…This means that there can be more than two liquid states for a single-component substance. Despite its counterintuitive nature, there have recently been many pieces of experimental and numerical evidence for the existence of LLT, for various liquids such as water (1)(2)(3)(4)(5), aqueous solutions (6)(7)(8), triphenyl phosphite (9-12), l-butanol (13), phosphorus (14), silicon (15,16), germanium (17), and Y 2 O 3 -Al 2 O 3 (18,19). This suggests that the LLT may be rather universally observed for various types of liquids.…”

“…This suggests that the LLT may be rather universally observed for various types of liquids. However, none of the LLTs reported so far is free from criticisms (20,21), mainly because these LLTs take place under experimentally difficult conditions [e.g., at high temperature and pressure (14,15,(17)(18)(19)] or in a supercooled state below the melting point (1-3, 5-7, 9, 10), where the transition is inevitably contaminated by microcrystal formation. The latter is not limited to experiments but arises in numerical simulations, often causing many controversies [LLT (22)(23)(24)(25) vs. crystallization (26)(27)(28)].…”