In this paper we describe for the first time an unusual phenomenon, occurring in FePO4 (TN = 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 TN (= 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 TN 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.
- Mössbauer spectroscopy
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics