RAMAN-STUDY OF AGREO4 IN THE SCHEELITE STRUCTURE UNDER PRESSURE

The Raman spectrum of AgReO4 was determined up to 18 GPa on a powdered sample in a diamond-anvil cell. The weak pressure dependence of the internal modes [especially of the symmetric stretching mode nu-l(A(g))] compared with the lattice modes suggests that the ReO4 tetrahedra are rather incompressible. At a reversible first-order phase transition at 13 +/- 1 GPa, discontinuities in the frequencies of all modes are observed. The determination of the Raman spectrum above 13 GPa was hindered by the persistence of the scheelite structure and the darkening of the sample. The Raman spectrum of the high-pressure phase is similar to that of the scheelite phase, except for the appearance of two modes in the gap of the frequency spectrum between the lattice and the internal modes of the scheelite phase. We were unable to determine the high-pressure structure.