Strong hydrogen-related electronic effects on the shear elastic constant ofTaV2H(D)x

Abstract

Resonant ultrasound spectroscopy has been used to measure the shear modulus of the C15 Laves-phase compounds TaV 2 H(D) x . Polycrystalline samples with x(H)ϭ0. 00, 0.06, 0.10, 0.18, 0.34, and 0.53 and x(D)ϭ0.17 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 ͑x͒. At 20 K, G(T) for TaV 2 H 0.53 was 55% greater than that of TaV 2 . For H concentrations of xр0.10, G(T) of TaV 2 H x shows anomalous stiffening with increasing temperature almost across the entire temperature range of study. For H concentrations xу0.18 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 TaV 2 H 0.10 and TaV 2 H 0.18 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 TaV 2 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.

Elastic properties of the Ta–V system: bcc Ta0.33V0.67and C15 TaV2

“…Figure 1 gives the polycrystalline shear modulus G ¼ C 44 of bcc Ta 0.33 V 0.67 as a function of temperature. Also shown for comparison are the results from the previous study of C15 TaV 2 [7]. Figure 2 gives the bulk modulus calculated from B ¼ C L À ð4=3ÞG (C L ¼ C 11 , polycrystalline).…”

“…However, the thermal expansion coefficient of C15 TaV 2 was found to be positive [2]; thus, the unusual temperature behaviour is likely due to the second term of equation (1). A mechanism based on electronic structure was proposed to explain the unusual behaviour found in a number of C15 materials [1,[3][4][5][6][7]. The C15 crystal structure gives doubly-degenerate electronic levels at the X point of the Brillouin zone.…”

“…This mechanism also accounted for the unusual effect of hydrogen on both the magnitude and temperature dependence of the elastic shear modulus of C15 TaV 2 H x . The addition of H alters the electronic structure, lifts the Fermi level away from the double-degeneracy point and gradually removes the anomalous behaviour as x is increased [7]. TaV 2 may be prepared as a C15 intermetallic compound or as a bcc random alloy [8,9].…”

Temperature dependence of the elastic moduli of polycrystalline LaAlxNi5−x and LaSnxNi5−x

“…The overall result, for typical materials, is that the elastic moduli approach 0 K with zero slope and decrease monotonically with increasing temperature. This simple picture is not applicable to materials undergoing phase transitions [22,23] or to materials with more complicated electronic structures [24][25][26]. Figs.…”

Phase transitions and thermally activated hydrogen dynamics in ZrV2Hx (0≤x≤1) intermetallic compounds

“…On the other hand, also other C15 Laves phases exhibit an anomalous softening, which has been related to the particular electronic properties of these compounds [11,12], and is influenced by the extra electrons supplied by H. Indeed, we observed that the increase of H concentration increases the temperature of the phase transformation and decreases the amplitude of the minimum in the modulus up to its disappearance in the ZrV 2 H 1 compound.…”