NMR evidence of two fractions of D atoms with different low-temperature mobilities in C15-type TaV2Dx and HfMo2Dx

Nuclear magnetic resonance measurements of the 2D spin-lattice relaxation rates in C15-type TaV2D0.5, TaV2D1.3 and HfMo2D0.2 have been performed at the frequency of 13.8 MHz over the temperature range 20–424 K. For all the samples studied, the relaxation rate is found to exhibit a peak near room temperature. This peak originates from the jump process leading to the long-range diffusion of D atoms. Below a certain temperature (130 K for TaV2Dx and 160 K for HfMo2D0.2) the spin-lattice relaxation becomes two-exponential indicating a coexistence of two fractions of D atoms with different low-temperature mobilities. The fast relaxation rate component shows an additional low-temperature peak; this component can be attributed to the fraction of D atoms participating in the fast localized motion with the jump rate τl−1. The unusual temperature dependence of τl−1 and the strong effect of H↔D substitution on τl−1 suggest that the localized motion is dominated by quantum effects.