Nuclear magnetic resonance study of hydrogen diffusion in HfV₂H_x(D_x) and ZrV₂H_x(D_x): effects of phase transitions and isotope substitution

Abstract

Nuclear magnetic resonance measurements of 1H and 2D spin-lattice relaxation times in HfV2Hx(Dx)(0.5<or=x<or=4) and ZrV2Hx(Dx)(1.1<or=x<or=5) have been performed over the temperature range 11-420 K. The experimental results are analysed to obtain the H(D) diffusion parameters in the disordered (high-temperature) and the ordered (low-temperature) phases. Hydrogen ordering is found to have strong effects on the diffusion parameters. In the ordered phases of HfV2Hx(Dx) and ZrV2Hx(Dx) the experimental data can be described in terms of the model using a double-peaked distribution of activation energies Ea for hydrogen motion. This model implies the coexistence of two types of H(D) motion with different frequency scales. For both systems at x>3.5 there is a distinct isotope effect in Ea for the disordered phase: EaD<EaH.

Phase transitions and thermally activated hydrogen dynamics in ZrV2Hx (0≤x≤1) intermetallic compounds

“…Indeed, our results concerns much lower H concentration, but is difficult to imagine a strong decrease of the activation energy for H jumping in less dilute solution. In addition, as we will show in the next paragraph, also from our measurements the activation energy for H dynamics in ordered phase turns out to be much lower than that for PH1 and comparable with the results of references [19,20]. As a consequence, a different mechanism should be considered for PH1.…”

“…The value obtained for the activation energy are similar to the result of Ref. [19] which obtain, from NMR measurements, Table 1 Parameters of the anelastic relaxations due to H in the ordered phase (in ZrV 2 H 0.7 ). For the definitions of the parameters, see the text However, due to the higher frequencies of their measurements the effect is shifted at higher temperature and occurs slightly above the transition temperature.…”

“…It is worth noting that the activation energies obtained from our results are sensibly higher with respect to those obtained for H diffusion in the disordered state by other authors [19,20] which are of the order of 0.2 eV or even lower for concentrations around x = 1. Indeed, our results concerns much lower H concentration, but is difficult to imagine a strong decrease of the activation energy for H jumping in less dilute solution.…”

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

“…Besides TaV 2 H x , two well-resolved R 1 (T ) peaks have been observed only for HfMo 2 H x [18] and Nb(V 1−y Cr y ) 2 H x (D x ) [19]. NMR data for other cubic Laves-phase hydrides including TiCr 1.8 H x [4], ZrCr 2 H x [20], HfV 2 H x and ZrV 2 H x [5,21] and ZrMo 2 H x [14] do not show any additional low-temperature R 1 peak; the localized H motion in these systems contributes only to the low-temperature slope of the single-peak R 1 (T ) dependence. In order to find out why the difference between the two frequency scales of H motion is most pronounced for TaV 2 H x , we may compare the ratios r 2 /r 1 in different Laves-phase hydrides.…”

“…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.…”