The π structure is a distorted 2 × 2 × 2 expansion of the eightatom rocksalt conventional cell, and differs significantly from the rhombohedral phase in that it retains a cubic P2 1 3 spacegroup but with a larger primitive cell with n a = 64 atoms. The Pnma structure has one unique Sn site with one short and two long bonds to chalcogen atoms (Table † ). In contrast, the π structure has four Sn sites, with 2 × 4 atoms in rhombohedral-like sites with three equivalent Sn-Ch distances and the other 2 × 12 in sites with three different Sn-Ch bond lengths.