Hydrogen diffusion in C15-type HfMo2H0.4: nuclear magnetic resonance evidence of two frequency scales of H hopping

Nuclear magnetic resonance measurements of the proton spin-lattice relaxati on rate in C15-type HfMo2H0.4 have been performed over the temperature rang e 20-420 K and the resonance frequency range 13-90 MHz. The temperature dep endence of the relaxation rate is found to exhibit two peaks. This is consi stent with a coexistence of two types of H hopping with different frequency scales. The experimental results are analysed to evaluate the parameters o f hydrogen motion and the electronic (Korringa) contribution to the relaxat ion rate. Our results support the idea that the ratio of the characteristic frequencies of the two types of H motion in C15 compounds AB(2) is related to the ratio of the metallic radii of elements A and B.