Strong Hydrogen-Related Electronic Effects On The Shear Elastic Constant Of Tav2h(D)(X)

Resonant ultrasound spectroscopy has been used to measure the shear modulus of the C15 Laves-phase compounds TaV2H(D)(x). Polycrystalline samples with x(H)=0.00, 0.06, 0.10, 0.18, 0.34, and 0.53 and x(D)=0.17 were investigated. Measurements were made over a temperature range of 3-345 K. Both the temperature dependence and the magnitude of the shear modulus [G(T)] were found to be highly dependent on the hydrogen concentration (x). At 20 K, G(T) for TaV2H0.53 was 55% greater than that of TaV2. For H concentrations of xless than or equal to0.10, G(T) of TaV2Hx shows anomalous stiffening with increasing temperature almost across the entire temperature range of study. For H concentrations xgreater than or equal to0.18 the temperature dependence of G(T) was reversed compared to that of the lower concentrations, exhibiting a more normal softening with increasing temperature. A minimum was found in G(T) for TaV2H0.10 and TaV2H0.18 at approximately 40 and 300 K, respectively. The results are in agreement with a model detailing electronic contributions to the single-crystal elastic constant c(44). The symmetry of the C15 structure results in doubly degenerate electronic energy levels at the X point of the Brillouin zone. These levels couple to an e(4) strain with resulting effects on c(44). The experimental results imply that the Fermi level of TaV2 lies very close to the double-degeneracy point. The effect of hydrogen is to raise the Fermi level above the double-degeneracy point with a resulting large change in the electronic contribution to c(44), which in turn affects the polycrystalline modulus G(T). The shift of the Fermi level with increasing hydrogen concentration determined from the elastic constant measurements is in remarkable agreement with the shift calculated from the electronic density of states available from other experiments. The results imply that each hydrogen atom contributes approximately one electron at the Fermi energy.