Structural phase transitions of HfV2 at low temperatures

We report a high-resolution synchrotron X-ray powder diffraction study on H fV2, hafnium divanadium, at low temperatures. In this work we show, for the first time, a complete sequence of structural phase transitions of HfV2 fr om cubic (Fd3m) to tetragonal (I4(1)/amd) to orthorhombic (Imma) in success ion as temperature decreases. Peak splitting and extra diffraction peaks ow ing to lattice distortion can be clearly distinguished for the low-symmetry phases. The atomic positions and lattice parameters were obtained by Rietv eld refinement. The bond lengths and angles of the HfV2 crystal structure a t the low-symmetry phases were correctly determined from the structure refi nement. The face-centered cubic (Fd3m) unit cell (Z = 24) transforms to a b ody-centered tetragonal (I4(1)/amd) phase with a 45 degrees rotation relati ve to the cubic cell and with a reduced number of atoms (Z = 12) in the uni t cell at a temperature of T = 112 K. The orthorhombic phase occurs at T = 102 K and it keeps the body-centered symmetry (Imma) and Z = 12 in the unit cell. The refinement results indicate that there may be a small amount of untransformed cubic phase left over in the lower symmetry phases. The abnor mal thermal contraction of both tetragonal phase and orthorhombic phase mar ks the significance of structural change in HfV2.