Structural Transformations in Three-Dimensional Crystalline GeSe2 at High Pressures and High Temperatures

Germanium diselenide GeSe2 is studied in situ at the pressure range 2-6 GPa using angle-dispersive X-ray diffraction in a large volume Paris-Edinburgh cell. The structure at 2 GPa and 698 K, solved with direct methods in the I space group (a=5.5073 (3) ?, c=9.9374(8) ?, Z=4), is three dimensional with all the GeSe4 tetrahedra sharing their corners. Anomalous lattice expansion along the c axis with increasing pressure and pressureinduced structural variations are explained by anisotropic lattice distortions due to cooperative tiltings of the rigid GeSe4 units.