Composition dependence of the elastic constants of β-phase and (α + β)-phase PdH_x

We have measured the composition and temperature dependence of the shear moduli C′ and C₄₄ for two-phase (α + β)- and single-phase β-PdH_x. In the two-phase region, the α- and β-phases are coherent. Here, the composition dependence of C₄₄ and C′ deviate negatively from a Vegard-type volume average. We attribute the deviations to two effects: (1) the partly in-series arrangement of the precipitate and matrix phases, relative to the externally applied stress, and (2) thermally activated anelastic relaxations involving the rapid motion of H interstitial atoms, leading to slight changes in the shape of coherent precipitates. The first effect is present for both C′ and C₄₄ and is temperature-independent, whereas the second is present only for C′ and is strongly temperature-dependent.