Pressure-induced structural transitions and valence change in TlReO₄: a high-pressure raman study

Abstract

The pressure dependence of the Raman-active vibrational modes in TlReO₄ 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, 1.9, 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 LiReO₄. From the large shift (more than 2 eV) in the optical absorption at the III-to-IV transition and from the absence of such a change in KReO₄ up to 25 GPa, we propose that the latter transition in TlReO₄ involves a change in the valence state of Tl to the Tl³⁺ state and that of Re to Re⁵⁺ to maintain charge neutrality. The latter valence states are known for Tl 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 TlReO₄ in the pressure range 20-30 GPa.