Structural investigation of temperature-induced phase transitions in HfV2O7

HfV2O7 powders were prepared by means of thermal decomposition of a mixture of vanadyl and hafnyl oxalates in air at 973 K. The room temperature struc ture and its evolution with temperature was investigated using both electro n microscopy (TEM and HREM) and powder diffraction (X-ray and neutron) tech niques combined with DSC measurements. HfV2O7 exhibits two temperature-indu ced phase transitions between room temperature and 480 K. Over the highest transition (370 K), HfV2O7 crystallizes in a cubic system with a cell param eter of 8.75 Angstrom and Pa(3) over bar space group. A negative thermal ex pansion is observed in this high temperature phase and can be correlated to anisotropic oxygen atom displacements. On the other hand, the room tempera ture structure of HfV2O7 is also described in a cubic symmetry, but the exi stence of superstructure reflections leads to a cell parameter three times larger than for the high temperature structure (similar to 26.25 Angstrom) Moreover, HREM calculations have shown that the atomic positions, usually u sed to describe the superstructures of the ZrV2O7 compounds, cannot be used in the case of HfV2O7 at room temperature.