Structure of ZrV 2O 7from -263 to 470°C

High-resolution neutron powder diffraction data were used to analyze the structure of ZrV ₂O ₇at seven temperatures from -263 to 470°C. The structure of ZrV ₂O ₇at 236°C was also refined using X-ray powder diffraction data. The thermal expansion is positive below about 100°C but becomes strongly negative above this temperature. Data from 88 to 470°C were refined using a 8.8 Å cubic Z=4 cell in space group Paoverline3. All V-O-V angles are 180° on average in this structure. Data from -263 to 66°C were refined with a 26.3 Å, Z=108 cell still in space group Paoverline3. 89% of the V-O-V linkages are bent to about 160° but 11% remain on three-fold axes and are therefore constrained by symmetry to be 180° on average. The strong negative thermal expansion of ZrV ₂O ₇above 100°C is attributed to the transverse thermal motion of oxygen atoms in the metal-oxygen-metal linkages. These thermal motions can occur only by changing the shape of the polyhedra; thus, the occurrence of negative thermal expansion for cubic AM₂O ₇compounds is strongly correlated with the rigidity of the AO ₆octahedra and MO ₄tetrahedra.