Nuclear magnetic resonance study of hydrogen diffusion in A15-type Nb₃AlH_x

Abstract

Nuclear magnetic resonance measurements of the proton spin-lattice relaxation rate T1-1 for A15-type Nb3AlHx (x = 0.13, 1.77 and 2.75) have been performed over the temperature range 20-420 K and the resonance frequency range 13-90 MHz. The experimental results are analysed to evaluate the electronic (Korringa) contributions to T1-1 and the parameters of hydrogen diffusion. It is found that the electronic contribution to the relaxation rate decreases with increasing H content. The effective activation energies for H diffusion derived from the high-temperature relaxation rate data are 0.21 eV for x = 0.13 and 0.30 eV for x = 1.77 and 2.75. The behaviour of T1-1 at low temperatures (T<270 K for x = 1.77 and 2.75) suggests a coexistence of at least two frequency scales of H hopping. It is shown that the structure of the sublattice of interstitial d sites (partially occupied by hydrogen) is consistent with the coexistence of two characteristic jump rates for H atoms.

Quasielastic neutron scattering study of hydrogen motion in A15-typeNb3AlHx

“…For Nb 3 AlH 0.13 the R 1 maximum is observed at 285 K for 0 /2 = 40 MHz and at 325 K for 0 /2 = 90 MHz; 14 the corresponding values of d −1 are shown by circles in Fig. 12.…”

“…Because of a possible hydrogen desorption, measurements at T Ͼ 420 K have not been performed. However, it is interesting to compare the present QENS results with the NMR measurements of the proton spin-lattice relaxation rate R 1 for the same Nb 3 AlH x samples, 14 since the NMR measurements are usually sensitive to lower jump rates than the backscattering QENS measurements. The characteristic feature of the proton spin-lattice relaxation data in metal-hydrogen systems is the R 1 ͑T͒ maximum observed at the temperature at which 0 d Ϸ 1, where 0 /2 is the NMR frequency and d is the correlation time for the dipole-dipole interaction between nuclear spins.…”

“…The characteristic feature of the proton spin-lattice relaxation data in metal-hydrogen systems is the R 1 ͑T͒ maximum observed at the temperature at which 0 d Ϸ 1, where 0 /2 is the NMR frequency and d is the correlation time for the dipole-dipole interaction between nuclear spins. 34 For Nb 3 AlH x the proton spin-lattice relaxation rate is dominated by 1 H-93 Nb dipole-dipole interactions, 14 Fig. 12.…”

“…Nuclear magnetic resonance ͑NMR͒ studies of H mobility have been reported for the A15-type hydrides Ti 3 IrH x , 11 Ti 3 SbH x , 12 V 3 GaH x , 13 and Nb 3 AlH x . 14 While the results for Ti 3 SbH x , V 3 GaH x , and Nb 3 AlH x suggest complex diffusion mechanisms implying at least two frequency scales of H jumps, [12][13][14] the NMR data cannot give direct information on spatial aspects of hydrogen motion. Such information can be obtained from quasielastic neutron scattering ͑QENS͒ measurements probing both the temporal and spatial scales of atomic motion.…”

Nuclear spin relaxation and diffusion of hydrogen in the A15 compound Nb3AlHx

“…The diffusion of H in the cubic A15-type structure Nb 3 AlH x has been studied by nuclear magnetic resonance (NMR) [1] and by quasielastic neutron scattering (QENS) [2]. Nb 3 Al is an example of intermetallic A 3 B compounds with the A15 structure that can absorb large amounts of H, and the aim of the NMR and QENS studies was to understand the H diffusion mechanism in these compounds.…”