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 CaW${\mathrm{O}}_4$, SrW${\mathrm{O}}_4$, and BaW${\mathrm{O}}_4$ have been investigated to 90 kbar hydrostatic pressure, with the use of the diamond anvil cell. The ($\frac{d\nu }{\mathrm{dP}}$)'s are all positive, except in the case of the lowest-frequency external mode. In BaW${\mathrm{O}}_4$, 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 HgW${\mathrm{O}}_4$ structure. The previously noted phase transitions in CaW${\mathrm{O}}_4$ and SrW${\mathrm{O}}_4$ are not confirmed in the present study.