Germanium Disulfide and Diselenide : Phase Diagram and Polymorph

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Stability regions for polymorphs of germanium disulfide and diselenide are located in the respective pressure-temperature spaces covering 3-11 GPa and 400-900℃. Lattice forms and the d-spacings of all the appearing phases, except for GeSe_2-IV, are identified with known crystal structures by powder X-ray diffraction analysis. A transition series, GeS_2-I-III-IV, isotypically relates to that of GeSe_2-I-II-III, while no GeS_2-II-like phase appears in the-GeSe_2 phase diagram. The orthorhombic GeS_2-IV and the isotypic GeSe_2-III show an extinction rule, h+l=odd in the hOl reflections, which provides them, with a pseudorectangular hexagonal lattice composed of a distorted eightfold modified simple CdI_2-type structure. The lattice constants and the density of the orthorhombic GeS_2-IV are a=6.780(9)A, b=6.270(3)A, c=11.96(7)A and ρ_x=3.57 g/cm^3, and those of the isotypic GeSe_2-III are a=7.330(5)A, b=7.071(4)A, c=12.12(1)A andρ_x =4.88 g/cm^3, respectively.
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