Pressure induced phase transitions in alkaline earth tungstates

We present a Raman study of the pressure induced structural phase transitions in the tungstates CaWO4 and SrWO4. Raman spectra up to 24 GPa at room temperature reveal reversible pressure induced phase transitions 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, exhibits a softening: in the scheelite structure, while in the high-pressure phase its slope becomes positive. Above the phase transition the modes originating from the antisymmetric v(3)(E(g)) and v(3)(B-g) stretching modes of the WO4 group exhibit a frequency jump to higher and lower 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.