Pressure-induced amorphization of antiferromagnetic FePO₄

In this paper we describe for the first time an unusual phenomenon, occurring in FePO₄ (T_N = 25 K), where pressure drives the crystalline low-pressure phase (I) into two, coexisting antiferromagnetic states; one amorphous designated as IIa, the other crystalline (IIb) with an enhanced coordination number. This is unlike the case of berlinite (AlPO₄), which completely amorphizes above 15 GPa. Measurements were carried out with Mössbauer Spectroscopy (MS) and X-ray diffraction (XRD) at CHESS, over the pressure range 0-30 GPa. XRD shows that the double transformation starts at ∼ 2 GPa reaching saturation at 7 GPa. MS, however, shows that the FePO₄-I phase coexists to the highest pressure, indicating possible formation of clusters with sizes undetected by XRD. The abundance of the FePO₄ IIa and IIb phases are about equal. Both XRD and the new T_N (= 60 K) value obtained by MS, show that the FePO₄-IIa phase is isostructural to CrVO₄. No change is observed in the relative abundance of the three phases at P > 7 GPa in which the I-phase constitutes about 10% of the total. The T_N value of the FePO₄-II phases increases with increasing pressure, from 50 K at 2.5 GPa to 65 at 25 GPa. The pressure transitions at room temperature are not reversible; after pressure release to ambient value, the FePO₄-I is completely restored only after heat treatment in air at T = 700°C.