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 ω_Q=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 ω_Q=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’.