Abstract
Temperature-induced phase transitions in 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 Å and Å at 70 K and for the tetragonal high-temperature (HT) scheelite phase are Å, and Å at 400 K. The corresponding unit-cell volumes are 434 and 442 . For the orthorhombic phase , , , and at 293 K. The orthorhombic-LT-scheelite and the orthorhombic-HT-scheelite phase boundaries drawn on the basis of the at the transition point intersect at negative pressures in a diagram. This intersection appears to satisfy the criteria required for the occurrence of an "isolated point" in the 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.
- Received 20 July 1987
DOI:https://doi.org/10.1103/PhysRevB.37.664
©1988 American Physical Society