L Journal of Alloys and Compounds 253–254 (1997) 374–377 Hopping rates of H(D) bound to heavy interstitials in Ta as investigated by mechanical spectroscopy at MHz frequencies * L. Di Masso, A. Biscarini, B. Coluzzi, F.M. Mazzolai Department of Physics, University of Perugia, Perugia, Italy Abstract The stress-induced reorientation of O(N)–H(D) pairs has been investigated at MHz frequencies in Ta by propagating longitudinal ultrasonic pulses along a ,111. crystal lattice direction. The resulting anelastic relaxation is found to be composed of two processes which are attributed to two distinct configurations of the pair. A comparison of the transition rates of free and trapped H(D) atoms in Nb and Ta suggests that both processes observed in Ta involve transitions of H(D) atoms between equivalent nn sites, which are probably slightly displaced from normal tetrahedral sites (dis-1T↔dis-1T transitions). In Nb bound H(D) atoms should be in a state delocalized over two equivalent sites, probably slightly displaced from normal tetrahedral sites (dis-2T state), and the reorientation in this metal would take place via (dis-2T↔dis-2T) transitions. Keywords: Anelastic relaxation; Ultrasonic attenuation; Point defects interactions 1. Introduction hopping rates, however, have only been observed at low temperatures, since measurements in Ta containing H(D) It is well known that H(D) in refractory bcc metals Ta, and O(N) have only been carried out at kHz frequencies Nb and V can be trapped by the heavy interstitials O and and below. In view of their relevance ultrasonic measure- N. The diatomic complex I–H(D) (I stands here for O or ments were extended in this work to the MHz frequency N) forms an elastic dipole, which can be reoriented by an range. applied stress field, thus giving rise to an anelastic relaxation that has been extensively investigated in Nb [1–10] and to a lesser extent in Ta [1,2,11] and V [12,13]. 2. Experimental Attention has recently been paid also to TaNb alloys containing H and O [14,15]. The marz grade tantalum specimen used in this experi- In the course of the above investigations it has clearly ment was a cylinder of diameter F 56 mm and length been shown that hopping rates of free and bound H(D) do l 525.9 mm supplied by MRC. The axis of the cylinder not match over common temperature ranges. This mis- was oriented along a ,111. crystal direction. The single match has usually been attributed to partial dissociation of crystal was doped with O and N by annealing the sample 23 | the I–H(D) complex during the reorientation process at about 1000 8C in poor vacuum ( P 10 torr) for about 5 induced by an applied stress. More recently [15], however, 72 h. To facilitate H(D) absorption a thin Pd film was the assumption has been made that this mismatch may also subsequently evaporated on the lateral surface of the reflect differences in the state (localized / delocalized) of sample at a temperature of 950 8C. Under the above doping free and trapped H(D). This idea has originated from the conditions the equilibrium concentration of O and N is observation that the hopping rate of free H(D) is higher in expected to coincide with the terminal solubilities of these | Nb than in Ta while the reverse is found to be true for impurities in Ta at 1000 8C, that is, n 5I/Ta 0.02 at. 5 N,O trapped H(D). The existence of these differences in the Deuterium was introduced at a concentration n 5D/Ta5 D 0.008 at. by annealing the sample in a D atmosphere at 2 appropriate pressures and temperatures. After various series of ultrasonic measurements at low temperatures the * Corresponding author. specimen was outgassed from D and, then, repeatedly 0925-8388 / 97 / $17.00  1997 Elsevier Science S.A. All rights reserved PII S0925-8388(96)02919-2