Temperature-Induced Phase Transitions in T scrL ReO sub 4: A Raman Spectroscopic and X-ray Diffraction Study.

Temperature-induced phase transitions in T scrL Re O sub 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 200K and this is a first-order transition. At temperatures above 400K 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 sub o = 5. 798angstrom and c sub o = 12.920angstrom and for the tetragonal high temperature (HT) scheelite phase are a sub o = 5.761angstrom, and c sub o = 13.33angstrom. The corresponding unit cell volumes are 434( angstrom ) sup 3 and 442( angstrom ) sup 3. For the orthorhombic phase a sub o = 5.623, b sub o = 5.791, c sub o = 13.295 and V = 432 ( angstrom ) sup 3. The orthorhombic -LT-scheelite and the orthorhombic -HT-Scheelite phase boundaries drawn on a P-T diagram intersect at negative pressures. 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 where such a possibility is demonstrated.