Hydrogen motion in C14-type HfCr₂H_x: quasielastic neutron scattering and NMR studies

Abstract

In order to study the mechanism and parameters of hydrogen diffusion in the hexagonal (C14-type) Laves phase HfCr 2 , we have performed quasielastic neutron scattering (QENS) measurements in HfCr 2 H 0.74 and nuclear magnetic resonance (NMR) measurements of the proton spin-lattice relaxation rate in HfCr 2 H x (0.30 x 1.62) over the temperature range 11-424 K. It is found that the diffusive motion of hydrogen in this system can be described in terms of at least two jump processes: a fast localized H motion with the jump rate τ −1 l and a slower process with the rate τ −1 d associated with H jumps leading to longrange diffusion. In the interval 175-400 K, the temperature dependences of both τ −1 l and τ −1 d are reasonably described by an Arrhenius law. For HfCr 2 H 0.74 the corresponding activation energies derived from the QENS data are 122±7 meV for τ −1 l and 148 ± 7 meV for τ −1 d ; the ratio τ d /τ l at room temperature is close to 16. The long-range hydrogen mobility is found to decrease with increasing H content.

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

“…Above 250 K the temperature dependence of D can be satisfactorily described by the Arrhenius law with the preexponential factor D 0 = (7.0 ± 0.4) × 10 −5 cm 2 /s and the activation energy E a = 151 ± 2 meV. This value of the activation energy for the tracer diffusion coefficient in HfCr 2 H 0.4 appears to be close to the activation energies for the slower H jump rate −1 d derived from the QENS measurements for HfCr 2 H 0.74 (148 ± 7 meV) in the range 269-400 K [15] and from the proton spin-lattice relaxation measurements for HfCr 2 H 0.30 (138 meV) and HfCr 2 H 0.60 (147 meV) [15]. However, below 250 K the temperature dependence of D deviates markedly from the Arrhenius behavior (see Fig.…”

“…(3) with the interpolated experimental value D(174 K) ≈ 8.6 × 10 −9 cm 2 /s then yields L ≈ 2.78 Å. This value of the jump length appears to be considerably longer than the distance between the nearestneighbor tetrahedral sites in HfCr 2 H x (∼1.2 Å [15]). Such a feature is typical of H diffusion in Laves-phase hydrides [1].…”

“…For H diffusion in HfCr 2 H 0.4 , we may safely assume that f = 1, since only less than 4% of all available tetrahedral sites are occupied by hydrogen. Using the proton spin-lattice relaxation data for HfCr 2 H 0.3 [15] and the Bloembergen-Purcell-Pound model [6], we estimate that d ≈ 1.5 × 10 −8 s at T = 174 K. Eq. (3) with the interpolated experimental value D(174 K) ≈ 8.6 × 10 −9 cm 2 /s then yields L ≈ 2.78 Å.…”

“…The preparation of the HfCr2Hx, compounds was analogous to that described previously [15]. The hydrogen concentrations in the samples were determined volumetrically with the estimated accuracy of ±0.03 H atoms per formula unit.…”

“…For hydrogen absorption at room temperature, the maximum value of x is about 1.6. QENS and NMR measurements of the proton spin-lattice relaxation rates in HfCr 2 H x (0.30 ≤ x ≤ 1.62) [15] have shown that the diffusive motion of hydrogen in this system can be described in terms of at least two jump processes: a fast localized H motion within the hexagons formed by tetrahedral interstitial (Hf 2 Cr 2 ) sites and a slower process associated with H jumps leading to long range diffusion. The hydrogen jump rate for the slower process is found to increase with decreasing H content; therefore, samples with low x should be more suitable for diffusivity studies.…”