Localized hydrogen motion in C15-type TaV₂H_0.65: temperature dependence of the H jump rate

Abstract

In order to study the temperature dependence of the jump rate τ −1 l for the fast localized motion of hydrogen in cubic (C15-type) Laves-phase TaV 2 , we have performed time-of-flight quasielastic neutron scattering measurements for TaV 2 H 0.65 over the temperature range 12-290 K. The quasielastic line broadening corresponding to the localized H motion has been observed down to 70 K; the observability of the quasielastic line at lower temperatures is limited by the decrease in the fraction of H atoms participating in the fast localized motion. In the range 70-290 K the temperature dependence of τ −1 l is found to deviate from the Arrhenius behaviour. This temperature dependence can be reasonably well described by the exponential function exp(T /T 0 ) with T 0 = 220 K. The same function with T 0 of the order of 100 K also appears to describe τ −1 l (T ) in some other metallic systems showing fast localized hydrogen motion.

Hydrogen diffusivity and the anomalous transverse spin relaxation in C14-type HfCr2Hx

“…(2) is much weaker than the Arrhenius one at low temperatures. Similar temperature dependence of the H jump rate has been found for the localized H motion in C15-type TaV 2 H x [26][27][28] and in some other metallic systems showing incoherent hydrogen tunneling [3,28]. Although Eq.…”

A neutron scattering study of hydrogen dynamics in coarse-grained and nanostructured ZrCr2H3

“…As can be seen from this figure, the temperature dependence of τ −1 l is considerably weaker than that of τ −1 d ; moreover, τ −1 l (T ) deviates from Arrhenius behavior. Both these features are typical of the localized H motion in Laves-phase hydrides [19,34,37]. At room temperature, the jump rate τ −1 l for cg-ZrCr 2 H 3 is a factor of 27 higher than τ −1 d .…”

Random walk theory of1H spin relaxation in TaV2Hx

“…There are two jump paths from a g site to nearest neighbours on a hexagon and one jump path to a near neighbour on an adjacent hexagon. Nuclear spin relaxation [4][5][6] and QENS [7,8] studies in this system have shown the existence of two hydrogen jump rates corresponding to fast jumps 1 within hexagons and slower jumps 2 between hexagons. It has also been established that there is a temperature-dependent fraction 1− p of the hydrogen that do not participate in the 1 and 2 jumps.…”