Low-Temperature Phase of Sodium Priderite, NaxCrxTi8–xO16, with a Monoclinic Hollandite Structure

Crystalline and glassy GeSe2 have been subjected to pressures up to 7.7 GPa and temperatures up to 1100°C using a highpressure belt-type apparatus. Two high-pressure crystalline polymorphs of GeSe2 have been synthesized. The crystal structure of the tetragonal form of GeSe2 prepared at 3.0 GPa and 500°C, with space group I-42d and a=5.7307(4) and c= 9.691(1), was determined by means of powder X-ray diffraction data. The structure consists of GeSe4 tetrahedra that share corners in a three-dimensional network and has a considerably higher density than the two-dimensional structure of ambientpressure GeSe2 consisting of both edge- and corner-sharing tetrahedra. The IR and Raman spectra of tetragonal GeSe2 are discussed in relation to the structure of GeSe2 glass. An orthorhombic polymorph of GeSe2 was formed during quenching from 7.7 GPa and 500°C. Amorphous GeSe2 compressed at 250–300°C was observed to partly crystallize at 3.0 and 7.7 GPa.