The structural chemistry of the Group IV-VI oxides and chalcogenides is strongly influenced by the stereochemical activity of the valence ns 2 lone pair on the tetrel T 2+ cations. This gives rise to five structure types across the TCh series (T = Ge/Sn/Pb, Ch = S/Se/Te), viz. a high-symmetry cubic rocksalt phase (Fm 3m spacegroup), two low-symmetry orthorhombic phases (Pnma, Cmcm), and intermediate-symmetry rhombohedral and cubic π phases (R3m/P2 1 3) (Fig. ).