A high pressure Raman study of KReO sub 4, RbReO sub 4 and CsReO sub 4 to 25 GPa and pressure induced phase transitions.

The pressure dependence of the Raman spectrum of KReO sub 4, RbReO sub 4 and CsReO sub 4 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 KReO sub 4 and RbReO sub 4 and two transitions in CsReO sub 4 have been deduced. These transitions are found to occur at 7.5, 10 and 14 GPa in KReO sub 4, 1.6, 5.5 and 15 GPa in RbReO sub 4 and at 10 and 14 GPa in the case of CsReO sub 4. 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.