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

A. Jayaraman, G. A. Kourouklis, R. M. Fleming, and L. G. Van Uitert
Phys. Rev. B 37, 664(R) – Published 1 January 1988
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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 Å3 and 442 Å3. For the orthorhombic phase a0=5.623, b0=5.791, c0=13.295, and V=432 Å3 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 PT diagram. This intersection appears to satisfy the criteria required for the occurrence of an "isolated point" in the PT 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

Authors & Affiliations

A. Jayaraman, G. A. Kourouklis*, R. M. Fleming, and L. G. Van Uitert

  • AT&T Bell Laboratories, Murray Hill, New Jersey 07974

  • *Permanent address: Department of Physics, National Technical University Zografou Campus, Athens, GR15773, Greece.

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Vol. 37, Iss. 1 — 1 January 1988

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