Pressure-temperature phase diagram of SeO2.: Characterization of new phases

We have investigated SeO2 at high pressures and high temperatures. Two new phases (beta-SeO2 and gamma-SeO2) and the boundary separating them have been found, following experimental runs performed at pressures tip to 15 GPa and temperatures up to 820degreesC. The two phases crystallize in the orthorhombic system in space group Pmc2(1) (no. 26) with a = 5.0722(1) Angstrom, b = 4.4704(1) Angstrom, c = 7.5309(2) Angstrom, V = 170.760(9) Angstrom(3) and Z = 4 for the beta-phase, and with a = 5.0710(2) Angstrom, b = 4.4832(2) Angstrom, c = 14.9672(6) Angstrom, V = 340.27(3) Angstrom(3) and Z = 8 for the gamma-phase. Both phases are stable at ambient pressure and temperature below -30degreesC. At ambient temperature the phases return to the starting phase (alpha-SeO2) in a few days. We discuss our findings in relation to a previous report of in-situ measurements at high pressures and ambient temperature. (C) 2004 Elsevier Inc. All rights reserved.