Structural transformations in the As-Se system under high pressures and temperatures

The high pressure beta- and gamma-phases of As2Se3, which are metastable at normal conditions, were verified and characterized. The beta-phase of As2Se3 is observed after pressure-temperature treatment at 3 < p <5 GPa in the temperature interval 300 < T< 650 degrees C. It is crystallized according to As2Te3 structure type with the parameters a = 13.465 +/- 0.012 angstrom; b = 3.735 +/- 0.003 angstrom; c = 9.471 +/- 0.008 angstrom; C 2/m; Z = 4; D-x = 5.41 g/cm(3). The gamma-phase of As2Se3 was obtained after treatment at p = 7 GPa and T = 950 degrees C in the form of single crystals for the first time. The parameters of its unit cell are: a = 8.113 +/- 0.009 angstrom; c = 21.08 +/- 0.02 angstrom; D-x = 6.37 glcm(3); Z-6; SG R3m (No. 160) or R (3) over barm (No. 166). A new compound AsSe2 is obtained after treatment at p = 7 GPa and T = 1050 degrees C. This compound does not exist at normal conditions and is crystallized in accordance with the structure type of the high-pressure MoSe2 phase (a = 3.2918 angstrom; c = 19.3238 angstrom; R3m; D = 6.387 g/cm(3); Z = 3). The difference in the behaviors of As2S3 and As2Se3 structural analogs under pressure is discussed from the standpoint of crystal chemistry. (C) 2015 Elsevier B.V. All rights reserved.