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The phase stability of YbVO4 under pressure has been investigated using synchrotron based angle dispersive x-ray diffraction and Raman spectroscopic techniques up to 34.5 and 26.5 GPa, respectively. The results indicate that the compound transforms from the ambient pressure zircon structure to the scheelite structure above 5.9 GPa with 11.8% volume discontinuity. The coexistence of the two phases is observed over a large pressure range. At 15.8 GPa, the (011) peak of the scheelite phase develops asymmetry, and the pattern at further high pressures could be fitted to a fergusonite-type monoclinic structure. On reducing the pressure, the fergusonite phase reverses back to the scheelite phase; the latter phase could be recovered as a metastable phase at ambient pressure. The refined structural parameters along with the equation of state are given for various phases of YbVO4. Changes in the vibrational properties across these transitions, particularly across the scheelite↔fergusonite transition, have been investigated using Raman spectroscopy.

AbslracL Raman xattering investigations of lithium polasium sulphate at high pressures have revealed l h a l the sulphate tetrahedra are orientationally disordered in the highpressure phases. I n order la understand the nature of lhe disorder, the lemperature dependence of the relative intensities of Raman lines associaled with lhe sulphale inlernal mnds are investigated. l l ~l h e resulls suggest a frozen-in static disorder rather lhan dynamic equilibrium between the acupancics of different orientations The behaviour of lhe Raman linewidths up to a preuure of 2. 50 kbar indicates sucassively broader distributions of the sulphate orientations. A mechanism of amorphization involving disorder in cation sublattices induced the orientational disorder of sulphate is proposed.