Pressure-induced polyamorphic transition in CaAl2O4 glass

In situ high-pressure ultrasonic velocity measurements of CaAl₂O₄ glass reveal abrupt irreversible discontinuities in the elastic wave velocities at 8--10 GPa. Total structure factor and pair distribution functions measured by synchrotron x-ray diffraction show a rapid change in the intermediate range structure attributed to a rearrangement of calcium ions over this narrow pressure region. Atomistic models obtained from molecular dynamics simulations reveal that this intermediate range structure is explained by a transition of Ca--O void radius distribution from a bimodal distribution with peaks at 2. 1 and 2. 4 to a single distribution centered at 2. 1. These abrupt structural changes involving the rapid increase in elastic wave velocity are markedly different to the continuous transformations observed in conventional network-forming glasses, such as SiO₂.