1H NUCLEAR SPIN RELAXATION CAUSED BY DIFFUSION IN HfV2·Hx THE MODEL OF RANDOM WALK TO NEAREST NEIGHBOR SITES

The dominent factors affecting the nuclear spin relaxation of 1H in HfV2·Hx hydrides are dipolar couplings. The important theoretical problem is the calculation of spectral density functions for the fluctuation of the dipolar couplings caused by the diffusive motion of 1H in the host lattice. We extended Torrey's theory, based on the model of random walk to nearest neighbor sites for homonuclear dipolar interaction, to the case that both homo- and heteronuclear interactions are existent. Then we calculated the GHH(k, y) and GHV(k, y) for determining the spectral densities JqHH(ω) and JqHV(ω) (q = 0, 1,2) according to the structure constants and H occupations of HfV2 ·Hx obtained from neutron diffractions. Using our theory and data derived from previous work of others, we analyzed the activation energy and the attempt frequency of 1H on interstitial sites and found that our results are satisfactory as compared with the conclusion derived from the phenomenological theory.