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

Abstract

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 ($\frac{d\ensuremath{\nu}}{\mathrm{dP}}$)'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.