Hydrogen motion inC15-typeZrMo2Hx

Abstract

In order to determine the parameters of H motion in the cubic (C15-type͒ Laves phase ZrMo 2 , we have performed quasielastic neutron scattering measurements and nuclear-magnetic-resonance measurements of the proton spin-lattice relaxation rate on ZrMo 2 H x (0.3рxр1.0) in the temperature range 10-430 K. The diffusive motion of hydrogen in this system can be described in terms of two jump processes: the fast localized H motion within the hexagons formed by interstitial g (Zr 2 Mo 2 ) sites and the slower hopping from one hexagon to the other. The hopping rate l Ϫ1 of the faster process is found to be of the order of 10 10 s Ϫ1 at 300 K. The characteristic frequency d Ϫ1 of the slower process at the same temperature is about 6ϫ10 7 s Ϫ1

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

“…This has been confirmed by recent QENS experiments for ZrCr 2 H x [13], ZrMo 2 H x [14], HfMo 2 H x [15] and ZrV 2 H x [16,17]. However, in these systems the difference between the two frequency scales of H motion appears to be smaller than in TaV 2 H x .…”

“…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.…”

“…These results have been attributed [11] to the difference in the fractions of H and D atoms participating in the localized motion. In fact, QENS measurements [11,[13][14][15][16][17] indicate that only a certain fraction p of hydrogen atoms participate in the localized motion (on the appropriate experimental frequency scale) in Laves phases, and that this fraction decreases with decreasing T. The 51 V relaxation data for TaV 2 H x and TaV 2 D x with nearly the same x suggest that the value of p for D atoms is considerably higher than that for H atoms. The strong effect of H ↔ D substitution on the amplitude of the low-T host-metal R 1 peak, with a larger peak appearing in the deuterides, has also been found for 51 V relaxation in the ternary Laves-phase NbVCrH x (D x ) [19] and for 45 Sc relaxation in ␣-ScH x (D x ) [37,38].…”

“…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.…”

Dynamics of hydrogen in Pr2Fe17H4 and Pr2Fe17H5 studied by quasielastic neutron scattering

“…While such a hopping was previously observed in non-stoichiometric Laves phase compounds [12][13][14][15], it was surprising to see it in Pr 2 Fe 17 H 5 , where the repulsive interaction between the two H atoms populating a hexagon might be expected to hinder the jumps. We have suggested that the jumps of these two H atoms must be correlated (that is, the simultaneous jumps of two atoms in the same direction must occur) in order to reconcile the dynamics of hydrogen hopping in Pr 2 Fe 17 H 5 with the Switendick criterion of minimal hydrogen separation distance.…”