Pressure Induced Phase Transitions in TlReO4

This proposal seeks to understand the impact of pressure on the structural distortions that occur because of long-range ordering of the Tl 6s2 lone pairs in scheelite-type thallium rhenate (TlReO4). TlReO4 undergoes a re-entrant tetragonal-monoclinic-tetragonal phase transition on cooling from 450 K and it is reported to undergo three phase transitions as the pressure is increased to above 10 GPa. The first is to an unknown orthorhombic phase at around 1.0 GPa. The second transition is to wolframite-related monoclinic phase near 2.0 GPa, and third to a BaWO4 (II)-related monoclinic phase at about 10 GPa. This is in marked contrast to the behaviour of another scheelite with a lone pair containing cation, BiVO4, that displays a monoclinic to tetragonal phase transition both upon heating and on the application of pressure.