HIGH-PRESSURE STUDY OF PHASE-TRANSITIONS IN ALPHA-FEPO4

High pressure behaviour of FePO4 in berlinite form hits been investiga ted up to 10 GPa using vibrational Raman spectroscopy and energy dispe rsive x-ray diffraction. Combination of these techniques along with st udies on pressure quenched samples reveal structural transitions in th is material from its room pressure trigonal phase to a disordered and a crystalline phase near 3 +/- 0.5 GPa. The latter is the Cmcm phase w hich is the equilibrium structure at high pressures. These high pressu re phases do not revert back to its initial structure after release of pressure. Irreversibility of these transformations indicates that FeO 4 tetrahedra do not regain their initial coordination. These high pres sure transitions can be rationalized in terms of the three level free energy diagram for such systems.