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.