Pressure-induced concurrent transformation to an amorphous and crystalline phase in berlinite-type FePO₄

X-ray diffraction, Raman scattering, and Mössbauer 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 Fe^III and P^V ions, respectively, are sixfold and fourfold coordinated. In the 6-25 GPa 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.