Pressure-Induced Concurrent Transformation to an Amorphous and Crystalline Phase in Berlinite-TypeFePO4

Abstract

X-ray diffraction, Raman scattering, and Mössbauer spectroscopy provide a diverse description of the high pressure behavior of berlinite-type ${\mathrm{FePO}}_4$. At a pressure of 2.5(5) GPa, a transformation to a coexisting new crystalline $\left(\mathrm{chp}\right)$ and amorphous $\left(\mathrm{ahp}\right)$ phase is observed with about equal abundance. The $\mathrm{chp}$ phase is identified as a ${\mathrm{VCrO}}_4$ type, where ${\mathrm{Fe}}^{\mathrm{III}}$ and ${\mathrm{P}}^{\mathrm{V}}$ ions, respectively, are sixfold and fourfold coordinated. In the 6–25 GPa range and after decompression, the relative abundance of the $\mathrm{chp}$ and $\mathrm{ahp}$ 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 ${\mathrm{SiO}}_2$ analogs.