Pressure-induced cubic-to-orthorhombic phase transition in ZrW{sub 2}O{sub 8}
- Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States)
- Inorganic Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR (United Kingdom)
- Department of Chemistry, Oregon State University, Corvallis, Oregon 97331 (United States)
The crystal structure of ZrW{sub 2}O{sub 8} and its variation with pressure and temperature have been investigated by {ital in situ} neutron powder diffraction. At room temperature, the cubic {alpha} phase is stable below 0.21 GPa, where a first-order transition to the orthorhombic {gamma} phase, accompanied by a 4.95{percent} reduction in volume occurs. The transition involves the inversion of one third of the W{sub 2}O{sub 8} units, which is made possible by the migration of oxygen atoms that are bonded to only one W atom in the cubic phase. WO{sub 4} tetrahedra tilt off the threefold axes of the cubic cell and oxygen atoms that are coordinated to only one W atom in the cubic phase become coordinated to two W atoms in the orthorhombic phase. In spite of its smaller volume, the orthorhombic phase has a volume compressibility [({Delta}V/{Delta}P)/V={minus}1.53(1){times}10{sup {minus}2}thinspGPa{sup {minus}1}] that is slightly larger than that of the cubic phase [{minus}1.38(1){times}10{sup {minus}2}thinspGPa{sup {minus}1}]. This appears to result from a larger contribution of coordinated tilting of the ZrO{sub 6} octahedra and WO{sub n} polyhedra to the compression. The orthorhombic phase is retained upon release of pressure. Below room temperature, the metastable orthorhombic phase exhibits an average negative volume thermal expansion [({Delta}V/{Delta}T)/V] of {minus}3.4{times}10{sup {minus}6}thinspK{sup {minus}1}, which is an order of magnitude smaller than that for the cubic phase ({minus}2.6{times}10{sup {minus}5}thinspK{sup {minus}1}), apparently because of the reduced framework flexibility of the orthorhombic phase. Above room temperature, the thermal expansion of the orthorhombic phase becomes positive, prior to a first-order transition back to the cubic phase that occurs at about 390 K. {copyright} {ital 1999} {ital The American Physical Society}
- OSTI ID:
- 289193
- Journal Information:
- Physical Review, B: Condensed Matter, Journal Name: Physical Review, B: Condensed Matter Journal Issue: 1 Vol. 59; ISSN PRBMDO; ISSN 0163-1829
- Country of Publication:
- United States
- Language:
- English
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