Charge-transfer-enhanced prism-type local order in amorphous Mg65Cu25Y10: Short-to-medium-range structural evolution underlying liquid fragility and heat capacity

Abstract

Using classical and ab initio molecular dynamics simulations, we have probed into the atomic and electronic structures of an amorphous Mg alloy, Mg65Cu25Y10, as a representative of Mg alloys that form bulk metallic glasses (MGs). Different from some MGs where the icosahedral motifs are the key coordination polyhedra, here the featured short-range order (SRO) is dominated by Cu-centered bicapped square antiprisms and tricapped trigonal prisms. Bond shortening is observed for Mg–Cu and Y–Cu bonds, due to appreciable charge transfer that imparts an ionic character to the bonding. This enhances their chemical affinity, accentuating Cu-centered motifs analogous to solute-centered prisms in metal–metalloid MGs in this all-metal system. The prism-type SRO is prevalent even at high temperatures in the (supercooled) liquids, as revealed from the inherent structures. A weak temperature dependence is observed for the degree of characteristic SRO with undercooling, as well as for the development of connections of the motifs in the medium range. Such a structural evolution is contrasted with the rapidly ascending icosahedral order in Cu64Zr36 supercooled liquids, and explains the much more shallow specific heat curve as well as the low fragility of the Mg65Cu25Y10 supercooled liquid.