Structural phase transitions of HfV₂ at low temperatures

Abstract

We report a high-resolution synchrotron X-ray powder diffraction study on HfV(2), hafnium divanadium, at low temperatures. In this work we show, for the first time, a complete sequence of structural phase transitions of HfV(2) from cubic (Fd3m) to tetragonal (I4(1)/amd) to orthorhombic (Imma) in succession as temperature decreases. Peak splitting and extra diffraction peaks owing to lattice distortion can be clearly distinguished for the low-symmetry phases. The atomic positions and lattice parameters were obtained by Rietveld refinement. The bond lengths and angles of the HfV(2) crystal structure at the low-symmetry phases were correctly determined from the structure refinement. The face-centered cubic (Fd3m) unit cell (Z = 24) transforms to a body-centered tetragonal (I4(1)/amd) phase with a 45 degrees rotation relative to the cubic cell and with a reduced number of atoms (Z = 12) in the unit 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 abnormal thermal contraction of both tetragonal phase and orthorhombic phase marks the significance of structural change in HfV(2).

Structure and stability of Laves phases part II—structure type variations in binary and ternary systems

“…Well-known examples are HfV 2 [128][129][130][131][132][133] and ZrV 2 [134,135] which show a transformation from the C15 Laves phase structure to a tetragonal or orthorhombic (HfV 2 ) or a rhombohedral (ZrV 2 ) structure at about 110 to 115 K. Detailed investigations of the structural phase transformation in HfV 2 revealed that there is a complete sequence of transformations during cooling from the C15 Laves phase structure to a body-centered tetragonal structure at 112 K and to an orthorhombic structure at 102 K [133].…”

Yb6Ir5Ga7- A MgZn2Superstructure

“…Superstructure formation in such systems may be driven by electronic and geometrical factors as well as magnetic ordering or magnetostriction. Many examples have meanwhile been reported …”

A Proposal for a Composite with Temperature-Independent Thermophysical Properties: HfV2–HfV2O7

“…In comparison, several theoretical studies also predict cubic HfV2 to be energetically unstable in the ground state, reporting energies of formation between 20 and 35 meV per atom [29,74,75]. Furthermore, experiments show a transformation of cubic HfV2 upon cooling into an orthorhombic structure at around −160 °C [76,77] possibly due to kinetically limited decomposition into elemental V and Hf, since orthorhombic HfV2 also exhibits positive energy of formation [29,75]. However, for sputtered HfV2 at 500 °C (see Figure 2), the change in shape of the main peak at around 38° suggests the first formation of nanocrystals, and the emerging hump around 20° points towards intermetallic HfV2 nanocrystals, since their presence cannot be explained by hcp Hf or bcc V.…”

“…For ZrV 2 , due to the considerably higher energy of formation in the ground state [71], its energetic stabilization is expected at higher temperatures, potentially explaining the discussed phase formation differences between Hf-V and Zr-V thin films. For comparison, bulk synthesis of HfV 2 includes heat treatments at temperatures above 1200 • C [34,77,79]. The reduction in synthesis temperature to 700 • C is enabled by surface diffusion of adatoms during sputtering [80].…”