Polyamorphism in aluminate liquids

Supercooled liquids in the Y2O3-Al2O3 system undergo a liquid-liquid phase transition between a high-temperature, high-density amorphous polymorph (HDA form), and one with lower density that is stable at lower temperature (LDA form). The two amorphous polymorphs have the same chemical composition, but they differ in their density (similar to4% density difference), and in their heat content (enthalpy) and entropy determined by calorimetry. Here we present new results of structural studies using neutron and high-energy x-ray diffraction to study the structural differences between high-density amorphous (HDA) and low-density amorphous (LDA) polyamorphs. The combined data sets show no large differences in the average nearest-neighbour Al-O or Y-O bond lengths or coordination numbers between the low- and high-density liquids. However, the data indicate that changes occur among the packing geometries and clustering of the Al-O and Y-O coordination polyhedra, i.e., within the second-nearest-neighbour shell defined by the metal-metal (i.e., Y-Y, Y-Al and Al-Al) interactions. Polarizable ion molecular dynamics simulations of Y2O3-Al2O3 liquids are used to help interpret the pair-correlation functions obtained from x-ray and neutron scattering data. Unexpectedly large density fluctuations are observed to occur during the simulation, that are interpreted as due to dynamic sampling of high- and low-density configurations within the single-phase liquid at temperatures above the critical point or phase transition line. Calculated partial radial distribution functions indicate that the primary differences between HDA and LDA configurations occur among the Y-Y correlations.