The distortion of two FePO₄polymorphs with high pressure

Both the trigonal (Berlinite-type, phase-I), and orthorhombic (CrVO4-type, phase-II) forms of FePO4 have been studied at high-pressure using neutron powder diffraction. Phase-II was prepared by a high-pressure, high-temperature synthetic route, and recovered to ambient conditions. We report the first experimental high-pressure structural study of this phase up to ∼8.4 GPa at room temperature. It is shown that with increasing pressure, the FeO6 octahedra become more regular and decrease in volume, while the PO4 tetrahedra become less regular and increase in volume. For phase-I, similar changes in volume are determined, though without changes in distortion. At ∼2 GPa a signature of amorphisation has been observed for phase-I with the appearance of broad phase-II reflections. To support the results of the high-pressure study, Raman spectroscopic and SQUID magnetometry studies have been performed.