Study of elementary processes of diffusion in compound crystals of the L1_2 and E2_1 structures by anelastic

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Several years ago we suggested that the mechanical relaxation effect observed in Ni_3A1 was caused by stress-induced diffusional jumps of anti-site Al atoms (occupying Ni atom sites), and since then we have pursued the problem by carrying out various experiments. In the first year of this investigation we examined the effect of deviation of composition from the stoichiometry (25 mol % Al) on the relaxation effect in Ni_3A1. We found that the relaxation rate did not depend on composition, but the relaxation strength was increased markedly as the composition exceeded the stoichiometry. These results are in excellent agreement with the predictions from the postulated mechanism.Experiments on Ni_3Ga were made in the second year, where a similar mechanical relaxation effect was discovered. The values of the relaxation rate and its temperature dependence indicate that the relaxation is due to anti-site Ga atoms. Assuming that the diffusion of Ga in Ni_3Ga occurs via the same mechanism as the diffusion of Al in Ni_3A1, we have evaluated the diffusion coefficient of Al in Ni_3Al and that of Ga in Ni_3Ga. The latter is close to the data in the literature, which were measured directly by radio-tracer experiments, indicating that the relaxation is in fact due to diffusion of anti-site Ga atoms. As a preparation for extending the study to another L1_2 compound Ir_3Nb, we have measured the chemical diffusion.Lastly, in order to measure the diffusion of oxygen ions in oxides of the E2_1 (cubic perovskite) structure by the same principle, we have prepared samples of (Ba, Sr)TiO_3. We will carry out experiments on these samples in due course.