Pressure-induced polyamorphism in a main-group metallic glass

The mechanism of pressure-induced amorphous-to-amorphous transitions (AATs) in metallic glasses (MGs) has been a subject of intense research. Most AATs in MGs were found in lanthanide-based alloys and shown to originate from $4f$ orbital delocalization. Recently, evidence of an unexpected AAT in the main-group Ca-Al MGs was reported without a satisfactory explanation. Here, based on the results of first-principles molecular dynamics calculations, the suggested AAT at 12--15 GPa in the $\mathrm{C}{\mathrm{a}}_{72.7}\mathrm{A}{\mathrm{l}}_{27.3}$ MG is confirmed. Contrary to the common belief that the coordinate of metallic glasses with close packing cannot be increased further, the coordination around Al atoms is found to increase suddenly at the transition as a consequence of atomic migration and the aggregation of Al atoms. This transition originates from pressure-enhanced bonding between Ca $3d$ and Al $3p$ orbitals and is confirmed by the good agreement on the predicted and measured electrical conductivities. The theoretical analysis not only uncovers a mechanism of pressure-induced AAT in main-group MGs, but it can be generalized to establish a different perspective to guide the understanding of transformation phenomena in compressed MGs.