Temperature-induced phase transitions in TlReO₄: a Raman spectroscopic and x-ray diffraction study

Abstract

Temperature-induced phase transitions in TlReO₄ have been investigated by Raman spectroscopy and by x-ray powder diffraction. The data reveal the occurrence of a hitherto unreported phase transition from orthorhombic to the tetragonal scheelite-type structure near 200 K and this is a first-order transition. At temperatures above 400 K the orthorhombic phase is known to transform to the scheelite-type structure. These phase changes present a unique situation. The lattice parameters of the low-temperature (LT) scheelite phase are a₀=5.798 Å and c₀=12.920 Å at 70 K and for the tetragonal high-temperature (HT) scheelite phase are a₀=5.761 Å, and c₀=13.33 Å at 400 K. The corresponding unit-cell volumes are 434 ų and 442 ų. For the orthorhombic phase a₀=5.623, b₀=5.791, c₀=13.295, and V=432 ų at 293 K. The orthorhombic-LT-scheelite and the orthorhombic-HT-scheelite phase boundaries drawn on the basis of the ΔV's at the transition point intersect at negative pressures in a P-T diagram. This intersection appears to satisfy the criteria required for the occurrence of an "isolated point" in the P-T plane; namely a point at which two first-order phase boundaries intersect to become a continuous transition. This is probably the first experimental indication for such a possibility.