Pressure-Induced Concurrent Transformation to an Amorphous and Crystalline Phase in Berlinite-Type FePO4

X-ray diffraction, Raman scattering, and Moessbauer spectroscopy provide a diverse description of the high pressure behavior of berlinite-type FePO4 . At a pressure of 2.5(5) GPa, a transformation to a coexisting new crystalline (chp) and amorphous (ahp) phase is observed with about equal abundance. The chp phase is identified as a VCrO4 type, where FeIII and PV ions, respectively, are sixfold and fourfold coordinated. In the 6 endash 25GPa range and after decompression, the relative abundance of the chp and ahp phases remains unchanged. These phenomena of concurrent amorphous and crystalline transformations at low hydrostatic pressure and stable abundance ratio over a large pressure range are unique in pressure-induced structural transformations of SiO2 analogs. copyright 1997 The American Physical Society