Phase transformation and negative thermal expansion in TaVO5

Two phase transformations of TaVO5 were observed by DSC and/or dilatometry measurements in the studied temperature range. X-ray diffraction and neutron powder diffraction structure refinements indicated a phase transformation at-14 °C from a monoclinic symmetry with space group P2 1/c to an orthorhombic symmetry with space group Pnma above this temperature. The rigid TaO6 octahedron in orthorhombic phase becomes nonregular at-14 °C, which results in the transition from Pnma to P2 1/c. TaVO5 was found to be a negative thermal expansion material above room temperature. The calculated volumetric thermal expansion coefficients (TECs) are-8.92 × 10-6 °C-1 in the range of 20-600 °C, and-2.19 × 10-5 °C-1 above 600 °C, respectively. The negative thermal expansion behavior is accounted for by the tilt of the TaO6 and VO4 polyhedra, where the shrinkage of the VO4 tetrahedra result in the increase of Ta-O-V angles on heating, while the angle of Ta-O1-Ta maintains at 180° in the framework