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Published online by Cambridge University Press: 03 March 2011
Ultrasonic propagation of longitudinal waves (CL mode) has been investigated as a function of temperature in a single crystal of a TaH0.51 hydride. Stepwise changes of the elastic constant CL and of the attenuation A have been observed in the vicinity of the β ↔ ∊ and ∊ ↔ α phase transitions. These changes occur over narrow temperature ranges corresponding to regions of coexistence of two phases. A relatively small temperature dependent softening is displayed by CL on approaching Tβ→∊ from the low temperature side. At the transition temperatures no divergency has been observed in the attenuation, which appears to originate from domain boundary motions, rather than from the transitions themselves. A determination of the H diffusion coefficient by a permeation method at high temperature supports a view that adiabatic tunneling is an effective mechanism even at temperatures as high as 1173 K.