A high-pressure Raman study of KReO₄, RbReO₄ and CsReO₄ to 25 GPa and pressure-induced phase transitions

Abstract

The pressure dependence of the Raman spectrum of KReO₄, RbReO₄ and CsReO₄ has been investigated to 25 GPa using the diamond anvil cell. From the changes in the Raman spectra the occurrence of three pressure-induced phase transitions in KReO₄ and RbReO₄ and two transitions in CsReO₄ have been deduced. These transitions are found to occur at 7.5, 10 and 14 GPa in KReO₄, 1.6, 5.5 and 15 GPa in RbReO₄, and at 10 and 14 GPa in the case of CsReO₄. The transition sequence with increasing pressure seems to be from scheelite→pseudoscheelite→wolframite→denser complex structure. These changes would involve a change in the structure from a tetragonal→orthorhombic→monoclinic→triclinic lattice, with increases in density at each transition. No drastic change in the optical absorption occurs in these alkali rehenates up to 25 GPa, the highest pressure to which these materials were investigated. This is in contrast with the behavior of TIReO₄ which turns black at the third pressure-induced phase transition near 10 GPa presumably due to a change in the valence state of Tl from +1 to the +3 state.