PRESSURE-INDUCED AMORPHIZATION OF ANTIFERROMAGNETIC FEPO4

In this paper we describe for the first time an unusual phenomenon occ urring in FePO4 (T-N = 25 K), where pressure drives the crystalline lo w-pressure phase (I) into two, coexisting antiferromagnetic states; on e amorphous designated as IIa, the other crystalline (IIb) with an enh anced coordination number. This is unlike the case of berlinite (AlPO4 ), which completely amorphizes above 15 GPa. Measurements were carried out with Mossbauer Spectroscopy (MS) and X-ray diffraction (XRD) at C HESS, over the pressure range 0-30 GPa. XRD shows that the double tran sformation starts at similar to 2 GPa reaching saturation at 7 GPa. MS , however, shows that the FePO4-I phase coexists to the highest pressu re, indicating possible formation of clusters with sizes undetected by XRD. The abundance of the FePO4 IIa and IIb phases are about equal. B oth XRD and the new T-N (= 60 K) value obtained by MS, show that the F ePO4-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-ph ase constitutes about 10% of the total. The T-N value of the FePO4-II phases increases with increasing pressure: from 50 It at 2.5 GPa to 65 at 25 GPa, The pressure transitions at room temperature are not rever sible; after pressure release to ambient value, the FePO4-I is complet ely restored only after heat treatment in air at T = 700 degrees C. (C ) 1998 Elsevier Science B.V. All rights reserved.