High-pressure phase transitions of M2O5 (M = V, Nb, Ta) and thermal stability of new polymorphs

The stability regions of various M2O5 (M = V, Nb, Ta) polymorphs were studied by quenching samples from 600-1300 degreesC at pressures in the range 5.0-8.0 GPa. Above 7.0 GPa, Nb2O5 and Ta2O5 were found to have a new polymorph (Z phase) in which the metal atoms are in sevenfold coordination. In addition, the samples contained the high-pressure polymorph B-M2O5 with the rutile-related structure. Differential thermal analysis at atmospheric pressure showed a weak, broad exotherm, indicating that the transformation Of Z-M2O5 into other phases begins at 100-150 degreesC and reaches completion at 400 degreesC in Nb2O5 and 550 degreesC in Ta2O5. At p 8.0 GPa and t > 750 degreesC, a new high-pressure phase B-V2O5, isostructural with B-Nb2O5, was identified (a 11.9640(6) Angstrom, b = 4.6986(3) Angstrom, c = 5.3249(3) Angstrom, beta = 104.338(4)degrees, V = 290.01 Angstrom (3), Z = 4, sp. gr. C2/c). At atmospheric pressure, B-V2O5 transforms into alpha -V2O5, with two strong exothermic peaks at 230 and 270 degreesC. Experiments in the pressure range 5.0-8.0 GPa confirmed that the high-pressure phase beta -V2O5 has a broad stability region.