Electric field gradient at the Hf site and the phase transition in HfV2

Abstract

The electric quadrupole interaction at the Hf site in the cubic HfV2 compound, which shows lattice instability when cooled below 120 K, has been studied using the 482 keV (5/2+) state in181Ta. The time differential perturbed angular correlation (TDPAC) studies involving (133+136)-482 keV γ-γ cascade at room temperature show that only a fraction of181Ta nuclei see a cubic symmetry while the remaining experience a weak, randomly oriented, electric quadrupole interaction described by a Gaussian frequency distribution with ωQ=6.1±1.0 Mrad/sec and the relative width δ=0.5. There is a dramatic change in TDPAC pattern indicating a phase transition when the compound is cooled down to 77 K. At this temperature the quadrupole interaction frequency is found to be ωQ=40.6±3.0 Mrad/sec with δ=0.35. The experimentally observed EFG both at the Hf and the V sites are compared to the theoretical estimates based on ‘point charge model’.