Nanoscale thermal stabilization via permutational premelting

Anomalous higher-than-bulk melting in some metal clusters has been previously ascribed to particularly compact cluster structures and emergent covalent bonding at small sizes. Using both classical and ab initio molecular dynamics incorporating realistic interacting atomic environments, we show a MgO cluster which, although structurally flexible and with bulklike ionic bonding, melts at a higher temperature than rock salt MgO. We propose that this unexpectedly high thermal stability is due to the premelted dynamics of a bi-isomeric fluxional state allowing facile permutation of ion positions, raising the configurational entropy and lowering the free energy. We argue that this entropic stabilizing effect could also occur in other (nano)systems which allow for atomic permutations with relatively little structural change.