PRESSURE-INDUCED PHASE-TRANSITIONS IN ALKALINE-EARTH TUNGSTATES

We present a Raman study of the pressure induced structural phase tran sitions in the tungstates CaWO4 and SrWO4. Raman spectra up to 24 GPa at room temperature reveal reversible pressure induced phase transitio ns at 10 and 11.5 GPa for CaWO4 and SrWO4, respectively. The lowest B- g mode, associated with the WO4-WO4 vibration along the c-axis, exhibi ts a softening: in the scheelite structure, while in the high-pressure phase its slope becomes positive. Above tile phase transition the mod es originating from the antisymmetric v(3)(E(g)) and v(3)(B-g) stretch ing modes of the WO4 group exhibit a frequency jump to higher and lowe r frequencies. In the high pressure phase the modes originating from v (3) show an unusual, strongly nonlinear softening in a small pressure region before becoming almost pressure independent.