Dependence of excess vibrational entropies on grain boundary structures in MgO: A first-principles lattice dynamics

Abstract

First-principles lattice dynamics calculations were performed to reveal an atomic-level origin of excess vibrational entropies at grain boundaries (GBs) in MgO. Fourteen symmetric tilt GBs with various structural units were systematically examined. The excess vibrational entropies were found to vary depending on the individual GBs, and as a result, the relative thermodynamic stability of the GBs studied changed with temperature. The excess GB volumes were less correlated with the excess vibrational entropies. By contrast, classifying ions in terms of their coordination numbers, bond-length changes at GBs were well correlated with atom-projected excess vibrational entropies for both Mg and O ions. Bond-length changes at GBs, which originate from changes in bond strength, are therefore a critical, well-defined descriptor for evaluating excess vibrational entropies and thereby excess free energies in MgO.