Pressure-Induced Structural Transitions and Valence Change in T scrL Re O sub 4: A High Pressure Raman Study.

The pressure dependence of the Raman active vibrational modes in T scrL ReO sub 4 has been investigated up to 16 GPa, using the diamond anvil cell. The abrupt changes in the Raman spectra indicate that three pressure-induced phase transitions occur at 0.5 GPa, 1.9 GPa and 9.7 GPa. The first two transitions are not accompanied by any significant change in the optical characteristics of the sample, but in the third transition at 9.7 GPa the sample turns black, exhibiting a deep red transmission in the very thin edges revealing that it is still an insulator. The transition to phase II we believe is subtle and probably involves a doubling of a crystal axis, while phase III is likely to have the same structure as LiReO sub 4. >From the large shift (more than 2 eV) in the optical absorption at the III to IV transition and the absence of such a change in KReO sub 4 up to 25 GPa we propose that the latter transition in T scrL ReO sub 4 involves a change in the valence state of T scrL to T scrL sup (3+) state and that of Re to Re sup (5+) to maintain charge neutrality. The latter valence states are known for T scrL and Re. Phase IV is a low symmetry structure and apparently the change in the ionic size triggers the structural transition. The change in the optical characteristics of phase IV suggests the possibility of a metallic state for T scrL ReO sub 4 in the pressure range 20-30 GPa.