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

Abstract

Temperature-induced phase transitions in TlReO 4 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 a 0 =5.798 A and c 0 =12.920 A at 70 K and for the tetragonal high-temperature (HT) scheelite phase are a 0 =5.761 A, and c 0 =13.33 A at 400 K. The corresponding unit-cell volumes are 434 A 3 and 442 A 3 . For the orthorhombic phase a 0 =5.623, b 0 =5.791, c 0 =13.295, and V=432 A 3 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 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 at which two first-order phase boundaries intersect to become a continuous transition. This is probably the first experimental indication for such a possibility.