X-ray and neutron diffraction studies and MD simulation of atomic configurations in polyamorphic Y2O3-Al2O3 systems

Supercooled liquids in the Y(2)O(3)-Al(2)O(3) system undergo a liquid-liquid phase transition between a high-temperature, high-density amorphous (HDA) polymorph form and one with lower density (LDA form) that is stable at lower temperature. The two amorphous polymorphs have the same chemical composition, but they differ in their density (ca. 4% 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 HDA and LDA polyamorphs. The combined datasets 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 substantial 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, Al-Al) interactions. Polarizable ion model molecular dynamics simulations of Y(2)O(3)-Al(2)O(3) 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 of nominally equilibrated systems. These are interpreted as being 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.