A feature of the group IV-VI chalcogenides is the rich structural chemistry related to the stereochemical activity of the tetrel lone pairs. Under ambient conditions, GeS and GeSe both adopt the "pseudo 2D" layered Herzenbergite structure formed by SnS and SnSe with the orthorhombic Pnma spacegroup (Fig. ). Pnma GeSe is also known to transform to the rhombohedral phase (space group R3m; Fig. )) adopted by GeTe under low hydrostatic pressure. GeSe has also been proposed to form the cubic rocksalt structure (space group Fm 3m; Fig. )) adopted by the Pb chalcogenides at elevated temperature and under pressure. A variety of other monochalcogenide phases are also reported, but we focus here on the Pnma, R3m and rocksalt phases.