Density functional theory calculations of the turbostratically disordered compound[(SnSe)1+y]m(VS

Abstract

Among composite materials that layer constituent substances of nanoscale thicknesses, [(SnSe) 1+y ] m (VSe 2 ) n emerges as an example where the constituents retain incommensurate lattice structures. Perpendicular to the stacking direction, the system exhibits random translations and random rotations on average, i.e. turbostratic disorder, with local regions showing twelvefold diffraction patterns. Earlier theoretical work on these structures showed that combining density functional theory with an empirical treatment of the van der Waals interaction gave structural parameters in good agreement with experiment, but no attempt was made to examine the relative orientations. Here we approximate the extended system with one extended constituent and one finite constituent, which allows the treatment of all relative orientations on equal footing. The calculations show how the twelvefold periodicity follows from how the ions of the SnSe layer lock in with favored positions relative to the VSe 2 layer, and the associated energy scale supports arguments for the overall turbostratic disorder. The success of this approximation in describing the structural parameters of the extended [(SnSe) 1+y ] m (VSe 2 ) n system encourages its use for other properties and for other similar systems with other chemistries.