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Pressure-induced cubic-to-orthorhombic phase transition in ZrW 2 O 8

1999; American Physical Society; Volume: 59; Issue: 1 Linguagem: Inglês

10.1103/physrevb.59.215

1095-3795

J. D. Jorgensen, Z. Hu, S. Teslic, D. N. Argyriou, S. Short, John S. O. Evans, A.W. Sleight,

Microwave Dielectric Ceramics Synthesis

The crystal structure of ${\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ and its variation with pressure and temperature have been investigated by in situ neutron powder diffraction. At room temperature, the cubic \ensuremath{\alpha} phase is stable below 0.21 GPa, where a first-order transition to the orthorhombic \ensuremath{\gamma} phase, accompanied by a 4.95% reduction in volume occurs. The transition involves the inversion of one third of the ${\mathrm{W}}_{2}{\mathrm{O}}_{8}$ units, which is made possible by the migration of oxygen atoms that are bonded to only one W atom in the cubic phase. ${\mathrm{WO}}_{4}$ 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 $[(\ensuremath{\Delta}V/\ensuremath{\Delta}P)/V=\ensuremath{-}1.53(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{GPa}}^{\mathrm{\ensuremath{-}}1}]$ that is slightly larger than that of the cubic phase $[\ensuremath{-}1.38(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{GPa}}^{\mathrm{\ensuremath{-}}1}].$ This appears to result from a larger contribution of coordinated tilting of the ${\mathrm{ZrO}}_{6}$ octahedra and ${\mathrm{WO}}_{n}$ 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 $[(\ensuremath{\Delta}V/\ensuremath{\Delta}T)/V]$ of $\ensuremath{-}3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}{\mathrm{K}}^{\mathrm{\ensuremath{-}}1},$ which is an order of magnitude smaller than that for the cubic phase $(\ensuremath{-}2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}{\mathrm{K}}^{\mathrm{\ensuremath{-}}1}),$ 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.