Pressure-induced cubic-to-orthorhombic phase transition in ZrW2O8

The crystal structure of ZrW2O8 and its variation with pressure and tempera ture have been investigated by in situ neutron powder diffraction. At room temperature, the cubic ct phase is stable below 0.21 GPa, where a first-ord er transition to the orthorhombic gamma phase, accompanied by a 4.95% reduc tion in volume occurs. The transition involves the inversion of one third o f the W2O8 units, which is made possible by the migration of oxygen atoms t hat are bonded to only one W atom in the cubic phase. WO4 tetrahedra tilt o ff the threefold axes of the cubic cell and oxygen atoms that are coordinat ed 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= -1.53(1) x 10(-2) GPa(-1)] that is slightly larger than that of the cubic phase [-1.38(1) x 10(-2) GPa(-1)]. This appears to result from a larger contribution of coord inated tilting of the ZrO6 octahedra and WOn polyhedra to the compression. The orthorhombic phase is retained upon release of pressure. Below room tem perature, the metastable orthorhombic phase exhibits an average negative vo lume thermal expansion [(Delta V/Delta T)/V] of -3.4 x 10(-6) K-1, which is an order of magnitude smaller than that for the cubic phase (-2.6 x 10(-5) K-1), apparently because of the reduced framework flexibility of the ortho rhombic phase. Above room temperature, the thermal expansion of the orthorh ombic phase becomes positive, prior to a first-order transition back to the cubic phase that occurs at about 390 K. [S0163-1829(99)01401-0].