Ab initiomolecular dynamics simulations of the two-step melting of NaSn

Ab initio density-functional molecular dynamics simulations were performed in order to study the melting behavior of the NaSn alloy. This alloy exhibits a phase transition at 756 K to a high-temperature solid showing rotational disorder, which has been described as an assembly of rapidly rotating Sn 4 tetrahedral clusters surrounded by mobile Na cations. Various simulation temperatures and volumes were considered in order to distinguish the features of both solid β-NaSn and plastic α-NaSn phases. The results show that the ability of Na cations to diffuse is strongly linked to the deformation of Sn 4 tetrahedra. In addition, the α-NaSn phase seems to be characterized by a sizable degree of structural disorder, with frequent breaking of the Sn 4 tetrahedral units. The influence of the volume of the simulation cell on the dynamical behavior of the alloy was found to be very important, with increasing disorder as the volume was reduced. Finally, strong correlations between structural and electronic properties were found, with increased disorder leading to more metallic behavior.