Ultrasonic velocity and attenuation in a TaH 0.51 hydride single crystal
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Ultrasonic propagation of longitudinal waves (CL mode) has been investigated as a function of temperature in a single crystal of a TaH0.5i hydride. Stepwise changes of the elastic constant CL and of the attenuation A have been observed in the vicinity of the /3 e and e *-> a' 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 T^^^ 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.
I. INTRODUCTION Over the past few decades measurements of ultrasonic attenuation and velocity have extensively been used to investigate static and dynamic properties of phase transitions in solids.1"6 From this point of view, metal-hydrogen systems, which usually exhibit several ordered nonstoichiometric phases (see the example in Fig. I), 7 have to some extent been neglected. Actually, most of the ultrasonic investigations carried out so far in metal-hydrogen systems at MHz frequencies8"12 were aimed at characterization of the elastic behavior of the a and a' H solid solutions within temperature and H concentration ranges far from the boundaries of the coexistence regions. The only exceptions are investigations performed in the Nb-H(D) 13 " 15 and V-H 1 6 systems, which have also been studied in the vicinity of some phase transitions. In the present work attention was directed toward the yS *-* e and e a ' transitions in the Ta—H system using ultrasonic pulse propagation methods. In the course of this work the H diffusion coefficient was measured at high temperatures, where relatively few sets of data were available. A further test of such data seemed to be needed, as they provide the only evidence that adiabatic tunneling is effective in controlling H diffusion even at temperatures well above room temperature.17'18 These additional observations will also be reported and discussed. II. EXPERIMENTAL A single crystal of tantalum hydride (TaH0.5i) was prepared by equilibration of a 99.95% pure Ta single crystal, in the form of a cylinder of 5.25 mm radius 2434
J. Mater. Res., Vol. 9, No. 9, Sep 1994
(R) and 10.45 mm length (L) supplied by Johnson Matthey and Co., Ltd., with ultrapure H2 gas at gradually decreasing temperatures from 1173 down to 593 K in an all-metal sealed quartz furnace. Pressure changes were monitored by a Datametrics capacitance manometer. Table I summarizes all the physicochemical treatments undergone by the specimen. The axis of the cylinder was superimposed to a (110) crystallographic direction. The surfaces of the bases of the cylinder were polished flat and made parallel to within 10~5 rad. Prior to an
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