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B. Bandyophadhyay and S. Hayashi
[Phys. Rev. B, Vol. 60, No. 14, pp. 10302-10315, 1999]
Vanadium-hydrogen-deuterium (V-H-D) alloys with ([H]+[D])/[V]=0.8 have been studied by means of X-ray diffraction, thermal analysis, and 1H and 2H NMR. The crystal structures of VH0.82 and VH0.6D0.2 are body-centered-tetragonal (bct), while those of VH0.2D0.6 and VD0.81 are body-centered-cubic (bcc). VH0.4D0.4 consists of bct and bcc, demonstrating the existence of an immiscibility region. The phase transition from the deltaD phase to the aiphaD phase is observed in VD0.81 between 200 and 240 K. The temperature range of the similar phase transition becomes broadened and shifted toward the lower temperature as the [H]/[D] ratio increases. In VH0.82 no phase transition is observed in the temperature range between 120 and 295 K. In VH0.81, H atoms occupy octahedral (O) sites, and H diffusion takes place between the ordered O sites. In VH0.6D0.2, H and D atoms occupy both the O and tetrahedral (T) sites, and the H and D atoms in the T sites diffuse faster than those in the O sites do, as indicated by Fig. 1. The diffusion between the ordered O sites is confirmed by the residual 1H dipolar broadening and the 2H quadrupolar line shape. In VH0.2D0.6 and VD0.81, H and D atoms occupy the T sites. In the deltaD phase H and D atoms diffuse among the ordered T sites, and consequently the 2H quadrupolar line shape is preserved and the 2H relaxation is caused dominantly by the dipolar interaction. Around the phase transition temperature range, the diffusion changes drastically. In the aiphaD phase the diffusion takes place between the disordered T sites, and another motional mode is additionally present. Temperature and frequency dependences of 1H and 2H spin-lattice relaxation times can be described by modified Bloembergen-Purcell-Pound equations, suggesting that H and D atoms undergo some kind of correlated motions. Activation energies and mean residence times of H and D atoms have been estimated for each site.
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