ABSTRACT
The hexagonal mixed-anion solid solution Na2(CB9H10)(CB11H12) shows the highest room-temperature ionic conductivity among all known Na-ion conductors. To study the dynamical properties of this compound, we have measured the 1H and 23Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates in Na2(CB9H10)(CB11H12) over the temperature range of 80-435 K. It is found that the diffusive motion of Na+ ions can be described in terms of two jump processes: the fast localized motion within the pairs of tetrahedral interstitial sites of the hexagonal close-packed lattice formed by large anions and the slower jump process via octahedral sites leading to long-range diffusion. Below 350 K, the slower Na+ jump process is characterized by the activation energy of 353(11) meV. Although Na+ mobility in Na2(CB9H10)(CB11H12) found from our NMR experiments is higher than in other ionic conductors, it appears to be an order-of-magnitude lower than that expected on the basis of the conductivity measurements. This result suggests that the complex diffusion mechanism and/or correlations between Na+ jumps should be taken into account. The measured 1H spin-lattice relaxation rates for Na2(CB9H10)(CB11H12) are consistent with a coexistence of at least two anion reorientational jump processes occurring at different frequency scales. Near room temperature, both reorientational processes are found to be faster than the Na+ jump process responsible for the long-range diffusion.
ABSTRACT
The vibrational spectrum of hydrogen and the parameters of H jump motion in the rhombohedral Th(2)Zn(17)-type compound Ce(2)Fe(17)H(5) have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying interstitial Ce(2)Fe(2) sites participate in the fast localized jump motion over the hexagons formed by these tetrahedral sites. The H jump rate τ(-1) of this localized motion is found to change from 3.9 × 10(9) s(-1) at T = 140 K to 4.9 × 10(11) s(-1) at T = 350 K, and the temperature dependence of τ(-1) in the range 140-350 K is well described by the Arrhenius law with the activation energy of 103±3 meV. Our results suggest that the hydrogen jump rate in Th(2)Zn(17)-type compounds strongly increases with decreasing nearest-neighbor distance between the tetrahedral sites within the hexagons. Since each such hexagon in Ce(2)Fe(17)H(5) is populated by two hydrogen atoms, the jump motions of H atoms on the same hexagon should be correlated.
ABSTRACT
The vibrational spectra of hydrogen and parameters of H diffusion in the coarse-grained C15-type system ZrCr(2)H(3) and in nanostructured ZrCr(2)H(3) have been studied by means of inelastic and quasielastic neutron scattering. It is found that the diffusive motion of hydrogen in coarse-grained ZrCr(2)H(3) can be described in terms of at least two jump processes: a fast localized H motion with the jump rate τ(l)( - 1) over the hexagons formed by interstitial Zr(2)Cr(2) sites and a slower process with the rate τ(d)( - 1) associated with H jumps leading to long-range diffusion. While τ(d)( - 1)(T) in the range 250-380 K follows the Arrhenius law with the activation energy of 142 ± 4 meV, the temperature dependence of τ(l)( - 1) deviates from Arrhenius behavior. The nanostructured ZrCr(2)H(3) samples prepared by ball milling consist of C15-type grains and strongly distorted (amorphous-like) regions. H atoms in the strongly distorted regions are found to be immobile on the time scale of our experiments. The microscopic picture of H jump motion in the C15-type grains of the nanostructured samples is similar to that in coarse-grained ZrCr(2)H(3); however, the ball milling leads to a considerable decrease in the jump rate τ(d)( - 1).
ABSTRACT
In order to study the mechanism and parameters of H jump motion in the nonstoichiometric Nb carbides, we have performed quasielastic neutron scattering (QENS) measurements for NbC(0.71)H(0.28) over the temperature range 11- 475 K. Our results indicate that about 30% of H atoms in this system participate in a fast diffusive motion. The temperature dependence of the corresponding H jump rate in the range 298-475 K follows the Arrhenius law with an activation energy of 328 ± 9 meV. The Q dependence of the QENS data suggests that the observed jump motion corresponds to long-range diffusion of H atoms along chains of the off-centre sites in carbon vacancies.
ABSTRACT
In order to study the mobility of hydrogen in nanostructured Laves-phase hydrides, we have measured the proton nuclear magnetic resonance (NMR) spectra and the proton spin-lattice and spin-spin relaxation rates in two nanostructured systems prepared by ball milling: ZrCr(2)H(3) and TaV(2)H(1+δ). The proton NMR measurements have been performed at the resonance frequencies of 14, 23.8 and 90 MHz over the temperature ranges 11-424 K (for coarse-grained samples) and 11-384 K (for nanostructured samples). Hydrogen mobility in the ball-milled ZrCr(2)H(3) is found to decrease strongly with increasing milling time. The experimental data suggest that this effect is related to the growth of the fraction of highly distorted intergrain regions where H mobility is much lower than in the crystalline grains. For the nanostructured TaV(2)H(1+δ) system, the ball milling is found to lead to a slight decrease in the long-range H mobility and to a suppression of the fast localized H motion in the crystalline grains.
