The heavier PbCh form the rocksalt structure, in which the Pb 2+ cations adopt a centrosymmetric octahedral environment with six equal Pb-Ch bond lengths and the 6s 2 lone pairs are inactive. For GeS/GeSe and SnS/SnSe, the "strain" induced by placing the Ge 2+ /Sn 2+ in a centrosymmetric environment leads to an energetic preference for the orthorhombic Pnma structure, where the "pseudo-2D" layered structure and three-coordinate geometry allow the 4s 2 /5s 2 lone pairs to project into an interlayer void space. The R3m phase adopted by GeTe, and by SnTe at low temperature, lies between these extremes, with a cation off-centering along the rocksalt {111} direction giving three short and three long T-Te bonds and a partial relaxation of the strain in the highersymmetry phase.