High-pressure,high-temperature study of GeS2 and GeSe2

Phase transitions of the GeX₂ (X = S, Se) dichalcogenides have been studied at pressures of up to p ? 8 GPa and temperatures from 675 to 1375 K, and portions of their p-T phase diagrams have been constructed using our and previous experimental data. The crystal structure of the GeS₂-III phase has been refined by the Rietveld method (HgI₂ structure, P4₂/nmc, a = 3.46906(2) Å, c = 10.9745(1) Å, Z = 2, D _x = 3.438 g/cm³, R = 0.06). GeSe₂-III crystals have been grown for the first time at p ? 7 GPa in the temperature range 875–1275 K. The unit-cell parameters of GeSe₂-III (hex) are a = 6.468 ± 0.004 Å and c = 24.49 ± 0.10 Å (D _meas = 5.16 g/cm³, D _x = 5.18 g/cm³, Z = 12).