An ultrasonic study of stress-induced ordering of hydrogen in single-crystal palladium hydride

Self Cite

Ultrasonic measurements have been carried out on TaV 2 H(D) x over the temperature range of 15-335 K. Temperature-dependent attenuation peaks were observed with maxima near 250 K for measurement frequencies near 1 MHz. These peaks are believed to be due to the same process as responsible for maxima in NMR spin-lattice relaxation rates observed at higher frequencies and temperatures, and associated with long-range hydrogen diffusion. The present results for activation energies and attempt frequencies are in agreement with the NMR results in the temperature range of the spin-lattice relaxation rate measurements, but the ultrasonic data suggest the presence of two Arrhenius processes for the long-range motion below about 200 K. The most significant result of the present measurements concerns a possible second peak at low temperatures associated with the local motion found in NMR and neutron measurements. For the present results, such a peak was not found in TaV 2 H x , but there was an indication of such a peak in TaV 2 D x for temperatures below 50 K. These results suggest that the H hopping rate for the local motion in TaV 2 H(D) x remains well above the ultrasonic frequencies used throughout the temperature range of the measurements, but that the D hopping rate is somewhat lower.

Section: Resultsmentioning

confidence: 99%

An ultrasonic study of hydrogen (deuterium) motion in the C-15 Laves-phase compound

Foster

Leisure

Skripov

1999

Self Cite

“…In the Zener effect the site symmetry is such that anelastic relaxation is not expected for an isolated hydrogen; however, hydrogen-hydrogen interactions lower the site symmetry and lead to anelastic relaxation. This is the situation for the octahedral (O) site in elemental fcc metals [36][37][38][39][40][41] and the T site in elemental hexagonal metals [42][43][44][45][46]. Depending as it does on H-H interactions, the magnitude of the Zener effect [47,48] depends on the hydrogen concentration as x 2 (1 − x 2 ) if the occupancy of the sites is not far from random.…”

Section: Introductionmentioning

confidence: 99%

Ultrasonic attenuation and dispersion due to hydrogen motion in the C15 Laves-phase compound TaV₂H_x

Foster

Hightower

Leisure

et al. 2001

Self Cite

Hydrogen in the C15 Laves-phase material TaV2Hx has been studied by means of resonant ultrasound spectroscopy over the temperature range of 15-345 K for a series of hydrogen concentrations (x = 0.00-0.53). Ultrasonic loss peaks and frequency shifts (dispersion) associated with the hydrogen motion were observed, yielding parameters for the hydrogen motion. Hydrogen in these materials is known to occupy the tetrahedral g sites which form a series of interlinked hexagons. The ultrasonic results were associated with H hopping between g-site hexagons. The relaxation rates for x⩽0.18 were best described as a sum of two Arrhenius processes. For x = 0.34 and 0.53 only a single Arrhenius process was needed to fit the results, although the presence of a second Arrhenius mechanism could not be over-ruled. A single relaxation rate was sufficient to fit the data; a distribution of rates was not required. The magnitudes of the attenuation and dispersion depended linearly on the hydrogen concentration implying that it is the relaxation of isolated H atoms (the Snoek effect) that is responsible for the mechanical damping. The faster local motion of H reported from nuclear magnetic resonance measurements for motion within g-site hexagons was not observed in the present study. This suggests that the H hopping rate for the local motion remains above the ultrasonic frequencies over the temperature range of study, or perhaps that too few H atoms participate in the local motion.

“…Since the (interstitial) sites of such a lattice gas are either populated by interstitial atoms (component 1) or by vacancies (component 2), Zener relaxation can take place precisely as in a two-component substitutional alloy. Metalhydrogen systems are an interesting realization of such a lattice gas because of the high diffusivity of the hydrogen interstitials [5,6], so the Zener relaxation in these systems was 0953-8984/96/397233+15$19.50 c 1996 IOP Publishing Ltd intensively investigated, for instance in Pd [7][8][9][10][11][12] and in Sc and Y [13][14][15][16][17]. It is worth mentioning that a pure Zener relaxation was studied in these cases since the symmetry of the interstitial sites of the hydrogen (octahedral sites in face-centred cubic (fcc) Pd and tetrahedral ones in hexagonal close-packed (hcp) Sc and Y) excludes the occurrence of a Snoek relaxation.…”

Section: Introductionmentioning

confidence: 99%

A lattice gas model for the Zener relaxation

Wipf¹,

Kappesser²

1996

The Zener relaxation is an anelastic relaxation process in disordered crystals due to stress-induced changes of atomic order. We present a model calculation of this process for a simple cubic lattice gas which is non-interacting except for the exclusion of multiple occupancies. The relaxation results from the stress-induced formation and dissolution of bonds between (paired) lattice gas atoms on nearest-neighbour sites. Relaxation spectra are obtained for the compressibility s 11 +2s 12 and the shear compliance s 11 −s 12 . The spectra are proportional to c 2 (1−c) 2 , where c is the occupation probability of a given site. The frequency dependence of the spectra is determined by the jump rate of the lattice gas atoms. Compared with a spectrum for a single relaxation time, the spectrum of s 11 − s 12 is moderately broadened, whereas the spectrum of s 11 + 2s 12 shows a sizable broadening. The present results are applicable to both interstitial and substitutional alloys.