High pressure behavior of P2O5 crystalline modifications: compressibility, elastic properties and phase transitions

Solid phases of the P2O5 compound have rarely been studied under high pressure to date due to their record-high hygroscopicity. We present the results of in situ x-ray diffraction study of the structure of P2O5 molecular and polymeric modifications under pressures up to 11 GPa, as well as the results of an ultrasonic study of a molecular phase up to 2 GPa. At low pressures (0.3?0.7 GPa), there is a structural transformation in the molecular phase accompanied by a significant (7?8%) density increase. This phase transition is accompanied by a large increase of both the shear modulus (by 30?40%) and bulk modulus (by 20?25%). Compression curves of the molecular and polymeric covalent phases asymptotically converge to each other at 9?10 GPa and reach a density of 3.6?3.7 g cm?3. The polymeric modification is ?typical? for covalent oxides values of the bulk modulus and its derivative: B???35?40 GPa, B?P???3?4. The molecular phase has quite small values of the bulk and shear moduli (B???12 GPa, G???5 GPa).