PRESSURE-INDUCED CONCURRENT TRANSFORMATION TO AN AMORPHOUS AND CRYSTALLINE PHASE IN BERLINITE-TYPE FEPO4

X-ray diffraction, Raman scattering, and Mossbauer spectroscopy provid e a diverse description of the high pressure behavior of berlinite-typ e FePO4. At a pressure of 2.5(5) GPa, a transformation to a coexisting new crystalline (chp) and amorphous (ahp) phase is observed with abou t equal abundance. The chp phase is identified as a VCrO4 type, where Fe-III and P-V ions, respectively, are sixfold and fourfold coordinate d. In the 6-25 GPa range and after decompression, the relative abundan ce of the chp and ahp phases remains unchanged. These phenomena of con current amorphous and crystalline transformations at low hydrostatic p ressure and stable abundance ratio over a large pressure range are uni que in pressure-induced structural transformations of SiO2 analogs.