Pressure-induced cubic-to-orthorhombic phase transition inZrW2O8

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 α phase is stable below 0.21 GPa, where a first-order transition to the orthorhombic γ 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 $\left[\right(\Delta V/\Delta P)/V=-1.53(1)\mathrm{}\times {10}^{-2}{\mathrm{GPa}}^{\mathrm{-}1}]$ that is slightly larger than that of the cubic phase $\left[-1.38\right(1)\mathrm{}\times {10}^{-2}{\mathrm{GPa}}^{\mathrm{-}1}].$ 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 $\left[\right(\Delta V/\Delta T)/V]$ of $-3.4\times {10}^{-6}{\mathrm{K}}^{\mathrm{-}1},$ which is an order of magnitude smaller than that for the cubic phase $(-2.6\times {10}^{-5}{\mathrm{K}}^{\mathrm{-}1}),$ 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.