The high-pressure behaviour of the "moganite" polymorph of SiO2

The high-pressure behaviour of the "moganite" polymorph of SiO2 has been in vestigated by angle-dispersive, X-ray powder diffraction in a diamond anvil cell at room temperature. The bulk modulus was calculated from the P-V dat a: B-0 = 32.2 (3) GPa with B'(0) fixed to 5.2 as for quartz. The molar volu me of moganite becomes lower than the volume of quartz above 5 GPa. Pressur e-induced amorphisation occurred progressively; it was complete above 25 GP a. Upon decompression, amorphisation was partially reversible if the maximu m pressure reached was below 25-30 GPa and irreversible when the pressure r eached was above 45 GPa. The pressure-induced amorphisation of the quartz a nd moganite polymorphs of silica and phosphorus oxynitride PON, a silica an alogue, is related to the distortion of the tetrahedra and, in the case of silica, to the presence of a much denser phase, in which the cation has a h igher coordination number.