Stability of the Anomalous Large-Void CoSn Structure

The crystal structure of CoSn represents a highly peculiar intermetallic structure type because of the presence of a large void and highly nonspherically coordinated tin atoms. Voids in crystal structures of intermetallic compounds are most unusual and always give rise to speculations whether the observed phase is the ground state or not. Here we examine this problem by means of ab initio calculations and show that the void structure of CoSn is indeed the stable phase. With pressure we predict the following sequence of structural transformations: CoSn type → FeSi type → CsCl type. The structural properties of CoSn are understood in terms of competition between covalent and metallic bonding in the system.