High-pressure Raman study of alkaline-earth tungstates and a new pressure-induced phase transition in BaWO₄

The pressure dependence of the Raman peaks of the scheelite-type alkaline-earth tungstates CaWO4, SrWO4, and BaWO4 have been investigated to 90 kbar hydrostatic pressure, with the use of the diamond anvil cell. The (dν/dP)'s are all positive, except in the case of the lowest-frequency external mode. In BaWO4, a new reversible first-order phase transition occurs near 65 kbar, where the Raman peaks shift abruptly to lower frequencies. From the observed decrease in the W—O bond-stretching mode frequencies and the increased number of W—O bond-bending modes it is concluded that the oxygen coordination for tungsten becomes octahedral, and the phase transition itself is postulated to be to the monoclinic HgWO4 structure. The previously noted phase transitions in CaWO4 and SrWO4 are not confirmed in the present study