Nuclear magnetic resonance study of diffusion and localised motion of H(D) atoms in TaV₂H_x(D_x)

Abstract

Nuclear magnetic resonance measurements of 'H, 'D and "V spin-lattice relaxation rates T;' in TaV2H.JDI) (0.22 C x C 1.54) have been performed over the temperature range 10-440 K. For all the studied samples the double-peak temperature dependence of T;' is observed. The high-temperature relaxation rate maximum is associated with H(D) diffusion, and the low-temperature one is attributed to localised motion of H(D) atoms. The experimental results are analysed to obtain the parameters of the two types of motion as functions of temperature and H(D) content. At 300 K the diffusion rate is found to increase with increasing H(D) concentration. The characteristic period of the localised motion, bL, exhibits unusual temperature dependence, tLexp(-T/T,,) with To = 50 K.

Hydrogen jump motion in Laves-phase hydrides: Two frequency scales

“…On the other hand, Bowman et al [4] have suggested the possibility of a coexistence of two hydrogen jump processes with different rates. The unambiguous evidence for a coexistence of two types of hydrogen jump motion with different characteristic frequencies has been found in the series of NMR experiments on the C15-type TaV 2 -H(D) system [6][7][8][9]. These experiments have revealed the temperature dependence of the nuclear spin-lattice relaxation rate R 1 with two well-separated peaks.…”

“…Therefore, the discussion in this section will be based on the experimental results obtained for TaV 2 H x (D x ). Three interesting features of the localized hydrogen motion will be emphasized: (i) the strong dependence of the jump rate τ −1 l on hydrogen concentration, (ii) the unusual isotope effects on the parameters of the localized motion, and (iii) non-Arrhenius temperature dependence of the jump rate τ −1 l at low T. The hydrogen jump rate τ −1 l in TaV 2 H x (D x ) has been found to increase strongly with decreasing H(D) content [7][8][9]. In NMR measurements, this is reflected in the marked shift of the position of the low-T maximum of R 1 to lower temperatures.…”

“…Since for the quadrupole interaction only charge fluctuations are important, H and D atoms are expected to give the same contributions to the host-metal R 1 if their motional parameters are the same. The 51 V relaxation rate measurements for TaV 2 H x (D x ) [7] have revealed that the amplitude of the low-temperature R 1 peak for TaV 2 D x is nearly three times higher than that for TaV 2 H x with the same x. These results have been attributed [11] to the difference in the fractions of H and D atoms participating in the localized motion.…”

“…In NMR measurements, this is reflected in the marked shift of the position of the low-T maximum of R 1 to lower temperatures. For example, at the frequency of 90 MHz the low-T maximum of R 1 is observed at 187 K for TaV 2 H 1.33 [7], at 125 K for TaV 2 H 0.87 [7] and at 45 K for TaV 2 H 0.06 [9]. The strong dependence of τ −1 l on x has also been found in the QENS measurements on TaV 2 H x [11] and in the ultrasonic experiments on TaV 2 D x [35].…”

“…On the other hand, at room temperature both τ −1 l and τ −1 d can be measured (the former from the time-of-flight QENS and the latter from R 1 or the backscattering QENS). At present, the measured values of both τ −1 l and τ −1 d are available for five cubic Laves-phase systems with the exclusive g-site occupation and r 2 /r 1 > 1: TaV 2 H x [7,11], ZrCr 2 H x [13,29], ZrMo 2 H x [14], HfMo 2 H x [15,18] and ZrV 2 H x [16,17,21]. In Fig.…”