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The vibrational spectra of hydrogenated austenitic stainless steels AISI 304 (Fe/Cr18/Ni10) and AISI 310 (Fe/Cr25/Ni20) were investigated by inelastic neutron scattering. Based on the results of previous neutron diffraction studies, the data have been analysed by model calculations. The peaks corresponding to the optical modes show a significant broadening because of the variation of vibrational energies from one hydrogen site to another in the alloys. At high hydrogen contents (hydrogen-to-metal atomic ratios H/Me ≈1) the peak profile is dominated by hydrogen–hydrogen interactions. Correspondingly, the optical peaks show a splitting into longitudinal and transverse modes for H/Me ≈1. A continuous lowering of the vibrational energies with increasing hydrogen content was observed for both steels, reflecting a weakening of the hydrogen–metal interactions with increasing hydrogen–metal atomic distance.

We investigated the crystal structure and hydrogen vibrations in a hydrogen-loaded AISI 310 type austenitic steel (Fe-25Cr-20Ni) by neutron diffraction and inelastic neutron scattering. The samples with hydrogen content of H/Me = 0.43 and 0.92 were prepared in a hydrogen gas atmosphere at pressures up to 7.0 GPa. This doping procedure yields a more homogeneous hydrogen distribution within the sample than cathodic charging and allows clarifying the role of the concentration gradients and stresses on the phase transformation. Only fcc phase with increased lattice parameter was observed in both samples, however frequency spectra show a modification of the broad peak into two components at higher H content.

The chemical diffusion coefficient of D in V and of H and D in Ti were determined by Gorsky effect measurements at high H(D) concentrations. On V, data were taken between 22 and 70 °C for D/V ratios from 0.73 to 0.82 (pure bcc α phase). The diffusion coefficients follow Arrhenius laws with activation energies of ∼120 meV. They are extremely high, exceed in part 10 −4 cm 2 /s, and are up to four (two) times larger than the diffusion coefficients of D(H) in V at low concentrations. With the possible exception of Fe, they are the highest diffusion coefficients reported so far for H(D) in any metal in the considered temperature range, and probably also for any solid state diffusion process. The thermodynamic reason for the high diffusion coefficients is discussed. The measurements on Ti were performed between 160 and 310 °C for H(D)/Ti atom ratios between 1.5 and 1.71 (pure fcc γ phase). The diffusion coefficients are practically independent of concentration and follow Arrhenius laws with activation energies of 520 and 530 meV for H and D, respectively. H diffuses about 1.5 times faster than D. For the same concentration and isotope (H), the chemical diffusion coefficients in Ti are ten to fifty times larger than new and literature data for the self diffusion coefficient determined by field-gradient nuclear magnetic resonance and quasielastic neutron spectroscopy. These large differences result mainly from high values of the concentration derivative of the chemical potential of H in Ti.

Neutron spectroscopy on H-doped b.c.c. Nb0.8Mo0.2 alloys demonstrated recently (i) a complete suppression of low-temperature H precipitation up to 5 at.% H, and (ii) an exclusive H occupation of tetrahedral interstitial sites, like pure b.c.c. Nb. We studied by mechanical spectroscopy Nb0.8Mo0.2 alloys containing 0.85 and 3 at.% H and found a H-induced low-temperature relaxation at 2 kHz around 80 K. Activation energy and reciprocal pre-exponential relaxation time are ∼0.054 eV and ∼2.8·107 s−1, respectively. The relaxation peak is about 80% broader than for an ideal Debye relaxation, which reveals a spectrum of relaxation times. The height of the relaxation (Q−1) peaks is approximately linear to H concentration, thus indicating a Snoek-type relaxation of single H atoms. The heights of the peaks yield a tetragonality |λ1–λ2| of about 0.04, which is an order of magnitude smaller than that of the Snoek effect of O in Nb. The results suggest that the observed relaxation reflects low-temperature diffusive jumps of single H atoms between tetrahedral interstitial sites, with a jump rate above ∼1.2·104 s−1 at 80 K.

By neutron spectroscopy, we studied vibrations of H interstitials in two austenitic fcc steels (Fe0.55Cr0.20Mn0.10Ni0.15 and Fe0.54Cr0.27Ni0.19) doped with 0.37 and 0.33 at% H. The band modes, in which H vibrates with its metal neighbours, cause a weak intensity in the energy range of the acoustic vibrations of the H-free steels. The energies of the fundamental and the twofold local-mode excitations, in which H vibrates against its metal neighbours, were ~ 130 and ~ 260 meV, respectively. The respective peaks in the spectra were broadened because the metal neighbours of H, and thus its vibrational energies, vary from interstitial site to interstitial site. The above energy values support an H occupation of octahedral interstitial sites.

We studied by neutron spectroscopy, and for temperatures from 10 to 300 K, the local vibrations of hydrogen interstitials in two niobium-based alloys containing 5 and 20 at.% molybdenum (Nb 0.95 Mo 0.05 H 0.03 and Nb 0.8 Mo 0.2 H 0.05 ). Within experimental accuracy, the vibrational energies agree with those of hydrogen in pure niobium. This indicates that the hydrogen in the two alloys occupies tetrahedral interstitial sites. At 10 K, the values of the vibrational energies show a complete hydrogen precipitation in the Nb 0.95 Mo 0.05 H 0.03 alloy and no hydrogen precipitation in the Nb 0.8 Mo 0.2 H 0.05 alloy. The fact that the hydrogen in the latter alloy was still in solid solution means that the molybdenum alloy components lead to an increase of the solubility limit of the hydrogen.

Titanium and zirconium form hydrides TiH(D) x and ZrH(D) x with hydrogen concentrations between x ≈1.5 (Ti) or 1.6 (Zr) and x =2.0 (room temperature). In these hydrides, the metal atoms form a fcc (δ-phase) or a fct (ϵ-phase) lattice in which the hydrogen atoms occupy tetrahedral interstitial sites. All the tetrahedral sites are occupied at the maximum concentration x =2.0. The hydrogen atoms in titanium and zirconium represent a model system for a concentrated lattice gas. We studied hydrogen and deuterium diffusion in titanium and zirconium hydride by mechanical spectroscopy (vibrating reed technique, temperatures from 5 to 400 K, frequencies between 160 and 1300 Hz). The experiments yielded large hydrogen-induced Zener-relaxation peaks between 240 and 340 K from which the jump rates of the hydrogen interstitials were determined with the help of a theoretical model for the Zener relaxation in a concentrated lattice gas. The jump rates follow an Arrhenius relation with activation energies of 0.49±0.04 eV (H in titanium and zirconium), 0.60±0.04 eV (D in titanium) and 0.51±0.04 eV (D in zirconium). Extrapolation of the present jump rates to higher temperatures allows a comparison with diffusion data from previous high-temperature nuclear magnetic resonance and neutron-scattering measurements. The comparison yields a perfect agreement for titanium hydride, and a poor one for zirconium hydride. The poor agreement for zirconium hydride indicates differences in the microscopic diffusion mechanism between low and high temperatures, which do not exist in the case of titanium hydride.

By means of small-angle neutron scattering the microstructure of two nanocrystalline Pd samples (prepared by inert gas condensation) has been studied at room temperature in a Q-range from [0pt] \(\) to [0pt] \(\). An additional subsequent doping of the two samples with H as well as with D (concentrations < 4 at%) caused contrast variations that provided more detailed structural information. The measured scattering intensity was modeled by a Porod contribution from large heterogenities (e.g. pores) and a contribution from spherical grains with a log-normal distribution of their radii. To account for the presence of grain boundaries, the grains were considered to be surrounded by a shell with a reduced Pd density and a thickness half as large as the thickness of the grain boundaries. For the above model, the data of the H-doped, D-doped and undoped sample were simultaneously fitted with one single set of adjustable parameters. The fits yielded for the two samples volume-weighted mean grain radii of 10 nm and 13 nm. The values for the grain boundary thickness lie between 0.2 and 0.8 nm. Almost all of the H- and D-atoms are, at low hydrogen concentrations, located in the grain boundaries.

We studied, by neutron spectroscopy, local vibrations of hydrogen interstitials and hydrogen trapping by nitrogen interstitials in TaN0.006H0.003 and in a nitrogen-free TaH0.086 reference sample between 1.4 and 295 K. The measurements on TaH0.086 yielded vibrational energies E1 = (113±1) meV and (doubly degenerate) E2,3163 meV at 295 K where hydrogen is in solid solution, together with a higher energy E1 = (120±1) meV and essentially unchanged energies E2,3 at low temperatures (1.4 to 4.2 K) where hydrogen precipitates in a hydride phase. The energies agree with previous studies. The measurements on TaN0.006H0.003 yielded, at 295 K and at low temperatures, vibrational energies identical to those of the nitrogen-free sample at 295 K. The results show that the hydrogen interstitials are trapped by the nitrogen at low temperatures and do not precipitate. They show further that the trapped hydrogen interstitials occupy tetrahedral sites, i.e. the same type of site as occupied in the absence of traps.

The vibrational spectrum of fcc γ -MnH 0.41 synthesized under high pressure of gaseous hydrogen was studied by inelastic neutron scattering at 2 K in the range of energy transfers from 25 to 400 meV. The fundamental band of optical hydrogen vibrations consists of a peak at 111 meV with a broad shoulder towards higher energies, which extends up to about 140 meV. At higher energy transfers, the spectrum originates from multiphonon neutron scattering and exhibits approximately harmonic behaviour. The results are compared with the available data for other metal hydrides.

The quantum diffusion of hydrogen trapped by oxygen interstitials in niobium is well studied in the temperature range below 150 K. The studies demonstrate that the hydrogen is trapped by the oxygen impurity atoms and that the trapped hydrogen performs a rapid local diffusion process between two nearest-neighbour tetrahedral interstitial sites. With temperatures rising above 150 K, the probability that the hydrogen leaves the trap sites increases. The hydrogen dissociates from its original oxygen trapping atom and carries out a long-range diffusional motion before it is trapped again by an oxygen impurity. We studied this situation by neutron scattering in the temperature range from 100 to 300 K. In our experiment we found a continuous transition from local diffusion at low temperatures to a predominantly long-range diffusion at higher temperatures. This behaviour can successfully be described by a two-state model: one state describes the local diffusion of the hydrogen in the trapped state. The other state describes the long-range diffusion process. Each of these states has a characteristic lifetime, which is also a measure for the probability of the hydrogen to be in this state.

We studied the influence of coherency stresses on deuterium diffusion in a ${\mathrm{NbD}}_{0.33}$ crystal by quasielastic neutron scattering, discriminating coherent from incoherent scattering by neutron spin analysis $(454\ensuremath{\le}T\ensuremath{\le}504\mathrm{K})$. The diffusion coefficient $D$ of the deuterium interstitials, describing collective diffusion according to Fick's law, was derived from the coherent quasielastic scattering. The values of $D$ exceed published Gorsky effect results for the same quantity by factors of up to 30. We demonstrate quantitatively that the large differences in $D$ reflect the dissimilar influence of coherency stresses on the deuterium fluctuations respectively probed by neutron scattering and Gorsky effect.

A solid solution of hydrogen in -Mn, -MnHx with xD 0:073, was prepared at 623 K and a hydrogen pressure of 0.85 GPa. A profile analysis of the neutron diffraction patterns measured at 225 K and at 300 K showed that hydrogen in-MnH0:073 randomly occupies the interstitial positions 12e (0, 0, 0.538) of the -Mn host lattice. An inelastic neutron scattering study of -MnH0:073 at 90 K revealed a pronounced peak at 6.4 meV and a rather broad optical hydrogen band with peaks at 73, 105 and 123 meV. The splitting of the optical band into three different modes agrees with the low symmetry, 2, of the 12e hydrogen sites. A tentative interpretation is given for the peak at 6.4 meV.

Niobium-oxygen solid solutions were studied by x-ray diffraction and inelastic neutron scattering (INS). Niobium rods (diameter 10 mm, length 80 mm) were doped with oxygen in a high-vacuum induction furnace in an oxygen atmosphere. Laue diffraction showed that the Nb-O samples had a “bamboo” structure with a single crystal length ranging from 5 mm to 10 mm. Powder samples prepared from the rods were used to measure the lattice parameters. The observed increase of the lattice parameters due to interstitial oxygen is in agreement with literature data. The vibrational frequency distribution of NbO 0.026 was measured by a time-of-flight neutron spectrometer at room temperature, and at 200°C to increase the intensity of the energy gain INS in the region of the oxygen vibrations. In addition to the known vibrational mode of interstitial oxygen at ω 1,2 =45 meV, a high-energy mode at ω 3 =84.2 meV was observed. This mode corresponds to oxygen vibrations polarized along the short axes of the octahedral position.

YBa2Cu3Ox exhibits a second-order tetragonal-to-orthorhombic transformation, at least in wide temperature ranges. The order parameter is proportional to the orthorhombic strain or to the O population difference between (½ 0 0) and (0 ½ 0) sites in the Cu(1) plane. Quasi-static anelasticity experiments were performed at 100 and 120 °C on YBa2Cu3Ox samples with O concentrations x between 6.29 and 6.9. The experiments show a drastic increase of both relaxation strength and relaxation time near the tetragonal-to-orthorhombic transformation at x 6.4, thus demonstrating i) large amplitudes and ii) a (critical) slowing-down of the order parameter fluctuations.

The diffusion of H interstitials in nanocrystalline Pd was investigated by neutron spectroscopy (temperatures of 288 and 300 K, grain diameters between 7 and 25 nm, H concentrations of 2.9 and 3.7 at.%). The experiments indicate that a high fraction of the dissolved H is located in grain boundaries (this agrees with previous studies). They show further that the H atoms in the grain boundaries perform a diffusion process which (i) is partially locally restricted and (ii) exhibits jump rates that are up to a factor ∼100 higher than those of diffusion within the grains.

We studied the diffusion of H in hcp YH x single crystals ( x ≤0.1), by mechanical spectroscopy (vibrating-reed technique, 280 to 1750 Hz). In agreement with previous data from polycrystalline YH x , we find an anelastic relaxation peak at ∼280 K. Our single crystals show a large anisotropy in the relaxation strength which demonstrates that this peak results from a Zener relaxation of H pairs. Two types of H jumps determine the relaxation kinetics. Comparison with previous quasielastic neutron scattering data identifies the jumps that cause the relaxation, showing further that their jump rates follow, over ten orders of magnitude, an Arrhenius law with an activation energy of (0.60±0.03) eV.

The interstitial D sites in the D-doped A-15 compound Ti3IrDx with x = 0.70 and 3.63 were determined by neutron diffraction (space group Pm 3 n (No. 223)). For x = 0.70, we confirm the D occupation of sixfold d positions (6d positions) already found for other H- and D-doped A-15 compounds. The complete and sole occupation of these positions yields the concentration x = 3. Accordingly, we found for the sample with x = 3.63 the additional occupation of 24k (or nearly 24k) positions. Our study suggests in particular a structural mechanism that successively replaces 6d positions by two adjacent and simultaneously occupied 24k positions in order to allow an increase of the D concentration above x = 3.

Hydrogen Diffusion in nanocrystalline Palladium has been investigated by means of quasielastic neutron scattering (QENS) in the temperature range 220–300K within the α-phase. The QENS -results reveal broad and narrow line widths with the first ones being significantly broader than those obtained from the coarse gained PdH reference sample indicating the presence of a fast diffusion process within the grain boundaries. The narrow line width seems to emerge from the diffusion within the nanocrystalline grains. A large elastic contribution to the spectra reveals hints on hydrogen traps due to internal surfaces and defects. For comparison of effects emerging from the preparation process samples have been investigated obtained from different preparation techniques, i.e. pulsed electrodeposition (PED) and inert gas condensation (IGC).

In Pd, MgO precipitates with diameters in the nm range can be produced by internal oxidation of Pdmg x samples that are annealed in an O 2 atmosphere. The Pd-MgO interfaces can be decorated with excess oxygen, depending on the O 2 gas pressure in the final annealing procedure. We studied by mechanical spectroscopy the influence of hydrogen (H) interstitials on the anelastic behavior of internally oxidized Pd-1.9 at %MgO samples (with and without oxygen decoration of the Pd-MgO interfaces, temperature range 4 to 380 K, frequencies between 213 and 685 Hz). The H causes a broad relaxation peak with an activation energy of ∼ 0.15eV for the underlying relaxation process. The peak does not exist in ordinary (pure) Pd doped with H. We conclude that the relaxation peak results from H interstitials that are trapped in the interfacial regions between the MgO precipitates and the Pd. This supports the results of a recent electrochemical study which similarly reports a very effective trapping of H interstitials in these regions. Surprisingly, the relaxation peak was not found to depend on whether the interfacial regions were decorated with oxygen or not.

The diffusion of excess oxygen atoms in La2CuO4+δ, (0 < δ < 0.035) was studied by mechanical spectroscopy with a vibrating reed method (220 Hz

In the hydrogen (H) concentration range 1.6≤x≤1.8, TiHx exists in a δ phase where Ti forms a fcc lattice in which the H occupies tetrahedral interstitial sites. We studied the diffusion of the H in this phase by mechanical spectroscopy (temperatures from 5 to 370 K, vibrating reed technique, frequencies from 169 to 1 290 Hz). We find a large H-induced Zener-relaxation peak between 240 and 270 K (for the present frequencies) from which we determined the jump rate of the H interstitials with the help of a recent theoretical model for the Zener relaxation. The jump rate follows an Arrhenius relation with an activation enthalpy of (0.49 ± 0.04) eV. An extrapolation of the present results agrees well with previous high-temperature data from neutron spectroscopy and nuclear magnetic resonance. For concentrations x > 1.8, a continuous (ferroelastic) phase transition takes place below ~ 320 K, changing the fcc Ti lattice into fct (ε phase). The phase transition is accompanied by a drastic softening of Young's modulus which allows a very precise determination of the transition temperature.

We have performed a mechanical-spectroscopy study on hcp YHx single crystals (x ≤ 0.1, vibrating-reed technique, 280 to 1750 Hz). In agreement with previous data from polycrystalline YHx, we find an anelastic relaxation peak at ~ 280 K. Our single crystals show a large anisotropy in the relaxation strength which demonstrates that this peak results from a Zener relaxation of hydrogen (H) pairs. Two types of H jumps determine the relaxation kinetics. Comparison with previous QNS data identifies the jumps that cause the relaxation, showing further that their jump rates follow, over ten orders of magnitude, an Arrhenius law with an activation energy of (0.60 ± 0.03) eV.

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 and the shear compliance . The spectra are proportional to , 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 is moderately broadened, whereas the spectrum of shows a sizable broadening. The present results are applicable to both interstitial and substitutional alloys.

The short-range order of the D interstitials in two (α′ phase) VD x powder samples ( x = 0.75 and 0.82) was investigated at room temperature by diffuse neutron scattering. The study demonstrates a (within experimental accuracy complete) blocking of interstitial sites in the two nearest shells around a given D atom, a reduced occupation probability for the sites in the third shell and an increased occupation probability of the sites in the fourth, fifth and sixth shells. These results prove directly the existence of a strong short-range repulsion between the D atoms.

The influence of H and D on the internal friction of nanocrystalline Pd was studied in vibrating-reed experiments (temperatures between 5 and 250 K; frequencies between 300 and 2000 Hz, H and D concentrations of about 2 and about 4 at.%). The data demonstrate a strong H- or D-induced internal-friction peak at around 160 K. The comparison with literature results for H-doped ordinary coarse-grained or single-crystal Pd suggests that the peak is caused by a Snoek-Koster relaxation of H or D atoms in the grains of the nanocrystalline Pd. The data show further a relaxation at around 70 K which is probably due to relaxation processes of H or D atoms in the grain boundaries.

The vibrational excitations and the position of hydrogen in nanocrystalline palladium were investigated by means of inelastic neutron-scattering and H solubility measurements. The study focuses on the H concentration regime (

The long-range elastic interaction between the O atoms in the Cu(1) atomic plane is shown to contribute to the tetragonal-to-orthorhombic phase transition in YBa 2 Cu 3 O x . A quantitative calculation yields a contribution of ∼ 10 % to the total interaction energy that drives the phase transition. It is further suggested that the consideration of the long-range elastic interaction is of importance for a theoretical modeling of the O ordering effects in the various orthorhombic phases.

The internal friction spectra of (i) plastically deformed Y, (ii) undeformed H-doped YH0.1 and (iii) plastically deformed and H-doped YH0.1 samples were measured with the help of the vibrating reed technique (polycrystalline foils, 5 K ⩽ T ⩽ 320 K, frequencies f between 100 and 1300 Hz, plastic deformations between 5% and 25% due to rolling at room temperature). The deformed (undoped) Y samples exhibit an internal friction peak in the same temperature range as the undeformed doped YH0.1 samples. We found further that combined plastic deformation and H-doping leads to a stronger peak than does sole deformation or H doping. The possible mechanisms for the observed relaxation process are discussed.

The relaxation strength measured in bending experiments on anelastic foils can substantially differ from the standard result that is applicable in the case of long rods. We analyze the relaxation strength of foil-shaped samples, presenting quantitative results for the two limiting situations of a long and a short foil. For cubic crystals, we discuss the relaxation strengths of two well-known relaxation processes: the Snoek effect and the Gorsky effect. We consider the influence of the foil geometry on the diffusion coefficient (or relaxation time) obtained in Gorsky effect measurements. Our results for the relaxation strength and the diffusion coefficient in Gorsky effect experiments are a generalization of previous calculations which apply to only two special crystal orientations.

Nuclear magnetic resonance (NMR) measurements of the 51V and 3H spin-lattice relaxation times and the 51V Knight shifts in the A15-type compounds V3GaHx (x = 0, 0.5 and 1.67) have been performed over the temperature range 11–450 K. The 51V NMR data show that the density of electron states at the Fermi level decreases strongly with increasing x. For both samples with x > 0 the proton free-induction decay has two well-resolved components in the range 180–380 K, indicating the presence of mobile and static H atoms on the NMR frequency scale.

We investigated the short-range order of D interstitials in cubic ( alpha '-phase) VD0.75 by neutron diffraction (room temperature measurements, powder sample). The VDx system is particularly favourable for neutron scattering experiments studying the short-range order of hydrogen interstitials in metals since the coherent scattering cross section of D exceeds the coherent cross section of V by a factor of approximately=300. This allows a data analysis that assumes that only the D atoms contribute to the coherent scattering intensity. The measured diffuse scattering intensity shows a nearly complete blocking of interstitial sites in the two nearest shells around a given D atom, and a reduced occupation probability for the sites in the third shell. The results indicate the existence of a strong short-range and repulsive interaction between the D atoms.

The self-diffusion of H in the substoichiometric rare-earth hydride halide YBrH0.78 was investigated by incoherent quasielastic neutron scattering. YBrH0.78 exhibits a layered structure in which the H atoms occupy tetrahedral interstitial sites located in Y bilayers, which are sandwiched by Br bilayers. As this arrangement suppresses H transfer between different Y bilayers, the present study investigates an essentially two-dimensional H diffusion within a given Y bilayer. The self-diffusion coefficient of the H was determined to be between 750 and 900 K. At 900 K, its value is D=(1.2+or-0.4)*10-6 cm2 s-1. The temperature dependence can be described by an Arrhenius relation with an activation energy E=(0.48+or-0.12) eV. The momentum-transfer dependence of the measured spectra suggests the presence of noticeable correlation effects in the H diffusion.

The vibrational modes of hydrogen in fcc Fe-25Cr-20Ni stainless steel with a hydrogen content of 0.33 at.% were studied by neutron spectroscopy. Hydrogen doping was performed at 810 K in a hydrogen-gas atmosphere of 190 bar. Neutron spectra were taken at 2 K and 77 K with the spectrometer IN1-BeF (ILL, Grenoble). The spectra show the fundamental hydrogen vibration at ≈130 meV and the second harmonics at ≈260 meV. The frequencies are higher than in other fcc hydrides. In spite of the cubic symmetry of the octahedral hydrogen positions and the low hydrogen content, the inelastic hydrogen peak has a relatively large width and an asymmetric shape.

We studied the H vibrations in H-doped YBa2Cu3OxHy, (y=0.6) by neutron spectroscopy. Neutron spectra were taken at 10 and 14 K from both orthorhombic (x approximately=6.9) and tetragonal (x approximately=6.3) samples. The spectra exhibit large H-induced intensities in the energy range between 40 and 130 meV. In comparison to orthorhombic samples, the H vibrations in tetragonal samples are shifted to lower energies by approximately 11%. This shift is opposite to the spectral shift of the lattice vibrations between H-free orthorhombic and tetragonal YBa2Cu3Ox, frequently attributed to differences in the electron-phonon coupling. Our results show that any such differences in the electron-phonon coupling are not apparent in the H vibrations. The results also support previous suggestions that the H occupies sites in the close neighbourhood of the Cu(l) atomic planes.

The self-diffusion coefficient of hydrogen in the dihydrides TiHx and YHx was investigated by neutron spectroscopy (1.59 < x < 2.09, 350 °C < T < 950 °C). The two hydrides differ in that hydrogen in TiHx occupies solely tetrahedral interstitial sites, whereas a small percentage (< 10%) of the hydrogen in YHx is also located on octahedral sites. The measurements demonstrate that these (small) differences in the interstitial occupancies cause a distinctly different concentration dependence of the self-diffusion coefficient. For both investigated systems, the concentration dependence of the self-diffusion coefficient can quantitatively be described with the model assumptions of a concentrated lattice gas.

The spin-lattice relaxation time of hydrogen in Ti3IrHx was determined from pulsed nuclear magnetic resonance measurements carried out between 5 and 630 K (x=0.55, 3.0 and 3.5; 5 K

We have measured the thermal conductivity κ and the heat capacity C of Ti, TiH 0.82 , TiH 1.7 , and TiH 2 at 0.35 K ≤ T ≤ 4 K. Whereas κ(TiH 0.82 ) and κ(TiH 1.7 ) are substantially smaller than κ(Ti), we find κ(TiH 2 ) − 3 κ(Ti). This latter remarkable result may arise from the enhanced electronic density of states — which is shown by the heat capacity data — and from a rather large electronic mean free path in this stoichiometric compound.

Quasi-static anelastic experiments on YBa2Cu3Ox (x 6.45, T = 100 and 120 °C) demonstrate a relaxation process with a wide spectrum of activation enthalpies H for the relaxation times (H 1 eV, σH = (0.07 ± 0.01) eV) and an extremely large relaxation strength Δ = 0.75 ± 0.15. The large Δ identifies the process as an oxygen redistribution between differently oriented sites in the CuO plane. The relaxation process characterizes the rate-determining step in long-range oxygen diffusion.

The jump rate of a particle in quantum diffusion is theoretically expected to be proportional to the square of the tunnel splitting (or tunnel matrix element). This basic relationship is exemplified in neutron spectroscopic experiments on the tunnelling of trapped-H interstitials in Nb.