Polyamorphism in aluminate liquids

Abstract

Supercooled liquids in the Y2O3–Al2O3system undergo a liquid–liquid phase transition between a high-temperature,high-density amorphous polymorph (HDA form), and one with lower density thatis stable at lower temperature (LDA form). The two amorphous polymorphs havethe same chemical composition, but they differ in their density (∼4%density difference), and in their heat content (enthalpy) and entropy determinedby calorimetry. Here we present new results of structural studies usingneutron and high-energy x-ray diffraction to study the structural differencesbetween high-density amorphous (HDA) and low-density amorphous (LDA)polyamorphs. The combined data sets show no large differences in the averagenearest-neighbour Al–O or Y–O bond lengths or coordination numbers betweenthe low-and high-density liquids. However, the data indicate that changesoccur among the packing geometries and clustering of the Al–O and Y–Ocoordination polyhedra, i.e., within the second-nearest-neighbour shelldefined by the metal–metal (i.e., Y–Y, Y–Al and Al–Al) interactions.Polarizable ion molecular dynamics simulations of Y2O3–Al2O3liquids are used to help interpret the pair-correlation functions obtained fromx-ray and neutron scattering data. Unexpectedly large density fluctuations areobserved to occur during the simulation, that are interpreted as due to dynamicsampling of high-and low-density configurations within the single-phase liquid attemperatures above the critical point or phase transition line. Calculated partialradial distribution functions indicate that the primary differences betweenHDA and LDA configurations occur among the Y–Y correlations.