Hydrogen spillover in tungsten oxide bronzes as observed by broadband neutron spectroscopy

Funding: This research was funded by the UK Science and Technology Facilities Council via direct access proposal (RB1920033, VESUVIO).Hydrogen spillover is an elusive process, and its characterization, using experimental probes and ab initio modeling, poses a serious challenge. In this work, the nuclear quantum dynamics of hydrogen in a palladium-decorated cubic polymorph of tungsten oxide, Pd/cWO3, are characterized by the technique of neutron Compton scattering augmented by ab initio harmonic lattice modeling. The deeply penetrating nature of the neutron scattering process, the lack of spectroscopic selection rules, the inherent high sensitivity to hydrogen, the high energy and momentum resolution for hydrogen, and the mass selectivity of the technique render the neutron Compton scattering a very potent and unique tool for investigating the local dynamics of hydrogen species in bulk matrices. The total neutron Compton scattering response of hydrogen is described in terms of the hydrogen momentum distribution. The distribution is deconvoluted under the assumption of three pools of hydrogen with distinctly different nuclear quantum dynamical behavior: (i) hydrogen-terminated beta-palladium hydride, (ii) hydrogen in acid centers (OH+ groups) on the surface of the cubic phase of tungsten oxide, and (iii) quasi-free atomic hydrogen inside the saturated hydrogen bronze resulting from the spillover process. The ab initio modeling of lattice dynamics yields theoretical predictions for the values of the widths of proton momentum distributions in the first two hydrogen pools, which allows for obtaining the contribution and the width of the momentum distribution of the quasi-free atomic hydrogen resulting from the hydrogen spillover process. The analysis reveals that the local binding strength of the quasi-free hydrogen is characterized by the values of nuclear momentum distribution width, nuclear kinetic energy, and force constant of the underlying potential of the mean force close to those of free, unconstrained hydrogen atomic species in a gas of non-interacting particles described by the Maxwell–Boltzmann distribution. Moreover, this picture of the local dynamics of the quasi-free hydrogen is consistent with the proton polaron model of hydrogen-induced coloration of bulk hydrogenated WO3.Publisher PDFPeer reviewe

A combined deep inelastic neutron scattering andab initiolattice dynamics study of the hydride anion dynamics and bonding in La2LiHO3 oxyhydride

Kobayashi et al (6279) (Science 2016, 351) reported recently the existence of pure H- conductivity in the oxyhydride La2−x−y Sr x+y LiH1−x+y O3−y , while demonstrating its functionality through a prototype solid-state Ti/La2LiHO3/TiH2 battery. In this study, we probe the atomistic motion of La2LiHO3 obtained by the promising halide salt flux method, via a combination of deep inelastic neutron scattering (DINS) and ab initio lattice dynamics (LD) calculations verified by vibrational inelastic neutron spectroscopy (INS). We successfully describe the measured momentum distributions from DINS via our LD calculations, without observing any diffusion activation over the temperature range reported by Kobayashi et al. This observation is corroborated by model predictions from our LD study, which reveals that the hydride anions remain bound within a 3D-harmonic potential. We conclude that with the current synthesis parameters, the method produces a vacancy free lattice, and that a necessary ingredient for diffusive motion of H- is the presence of a large population of vacancies. Based on the harmonic prediction for the hydrogen kinetic energy, we derive a picture of the evolution of the effective bonding potential for the hydride anions, and link this to the dynamics associated with decomposition of the oxyhydride.publishedVersio

Nuclear dynamics in BaZr0.7Ce0.2Y0.1O3−δ proton conductor as observed by neutron diffraction and Compton scattering

Concurrent neutron Compton scattering (NCS) and neutron diffraction experiments at temperatures between 70 K and 300 K have been performed on proton-conducting hydrated BaZr0.7Ce0.2Y0.1O3−δ (BZCY72) fabricated by spark plasma sintering. A combined neutron data analysis, augmented with density functional theory modelling of lattice dynamics, has enabled, for the first time, a mass-selective appraisal of the combined thermal and nuclear quantum effect on nuclear dynamics and thermodynamic stability of this technologically important proton conducting perovskite oxide. The analysis suggests that the nuclear dynamics in hydrated BZCY72 is a result of a subtle interplay of harmonic, anharmonic and thermal effects, with the increased anharmonic character of the lattice dynamics above the orthorhombic to rhombohedral phase transition at 85 K. The anharmonic effect seems to be most pronounced in the case of oxygen and cerium. The analysis of the proton momentum distribution reveals that the concentration of the hydrogen in the BZCY72 lattice is constant across the orthorhombic to rhombohedral phase transition and further down to the room temperature. Moreover, the average hydrogen concentration obtained from our analysis of the mass-resolved neutron Compton scattering data seems to be commensurate with the total vacancy concentration in the BZCY72 framework. The calculation of the vibrational enthalpy of both phases allows obtaining the value of the enthalpy of the orthorhombic to the rhombohedral phase transition of −3.1 ± 1 kJ mol−1. Finally, our analysis of the nuclear kinetic energy of the proton obtained from NCS and the oxygen-oxygen distance distributions obtained from ND allows to conclude that BZCY72 in both the orthorhombic and rhombohedral phase at 70 K and 100 K respectively falls into the category of the KDP-type crystals where proton is probably under the influence of a double-well potential and forms hydrogen bonds of moderate strength. The obtained results have important ramifications for this technological important material

A combined deep inelastic neutron scattering andab initiolattice dynamics study of the hydride anion dynamics and bonding in La2LiHO3 oxyhydride

Kobayashi et al (6279) (Science 2016, 351) reported recently the existence of pure H- conductivity in the oxyhydride La2−x−y Sr x+y LiH1−x+y O3−y , while demonstrating its functionality through a prototype solid-state Ti/La2LiHO3/TiH2 battery. In this study, we probe the atomistic motion of La2LiHO3 obtained by the promising halide salt flux method, via a combination of deep inelastic neutron scattering (DINS) and ab initio lattice dynamics (LD) calculations verified by vibrational inelastic neutron spectroscopy (INS). We successfully describe the measured momentum distributions from DINS via our LD calculations, without observing any diffusion activation over the temperature range reported by Kobayashi et al. This observation is corroborated by model predictions from our LD study, which reveals that the hydride anions remain bound within a 3D-harmonic potential. We conclude that with the current synthesis parameters, the method produces a vacancy free lattice, and that a necessary ingredient for diffusive motion of H- is the presence of a large population of vacancies. Based on the harmonic prediction for the hydrogen kinetic energy, we derive a picture of the evolution of the effective bonding potential for the hydride anions, and link this to the dynamics associated with decomposition of the oxyhydride

The effective isotropy of the hydrogen local potential in biphenyl and other hydrocarbons

We present an experimental investigation of the hydrogen nuclear momentum distribution in biphenyl using deep inelastic neutron scattering. Our experimental results suggest that the local potential affecting hydrogen is both harmonic and isotropic within experimental uncertainties. This feature is interpreted as a consequence of the central limit theorem, whereby the three-dimensional momentum distribution is expected to become a purely Gaussian function as the number of independent vibrational modes in a system increases. We also performed ab initio phonon calculations on biphenyl and other saturated hydrocarbons, from methane to decane. From the results of the simulations, one can observe that the nuclear momentum distribution becomes more isotropic as the number of atoms and normal modes in the molecule increases. Moreover, the predicted theoretical anisotropy in biphenyl is clearly larger than in the experiment. The reason is that the total number of normal modes necessary to reproduce the experimental results is much larger than the number of normal modes encompassed by a single unit cell due to the presence of structural disorder and intermolecular interactions in the real crystal, as well as coupling of different normal modes. Finally, experimental data were collected, over a subset of detectors on the VESUVIO spectrometer at ISIS, with a novel setup to increase the count rate and signal-to-background ratio. We envision that such an optimized experimental setup can provide faster measurements and more stringent constraints for phonon calculations

Water confinement in faujasite cages : a deuteron NMR investigation in a wide temperature range. 1, Low temperature spectra

Deuteron NMR spectra were measured for D 2 O con fi ned in NaX, NaY, and DY faujasites with various D 2 O loadings at temperatures ranging from T =70Kto T = 200 K with the aim to study the molecular mobility of con fi ned water as a function of Si/Al ratio and loading. The recorded spectra were fi tted with linear combinations of representative spectral components. At low loading, with the number of water molecules per unit cell close to the abundance of sodium cations, a component related to π -jumps of water deuterons about the 2-fold symmetry axis dominated. For loadings at levels 3 times and 5 times higher than the initial loading level, Pake dublets due to rigid water deuterons dominated the recorded spectra. A set of the quadrupole coupling constant values of localized water deuterons was derived from the analysis of the Pake dublets. Their values were attributed to deuteron positions corresponding to the locations at oxygen atoms in the faujasite framework and locations within hydrogen-bonded water clusters inside faujasite cages. The contributions of the di ff erent spectral components were observed to change with increasing temperature according to the Arrhenius law with a characteristic dynamic crossover point at T = 165 K. Below T = 165 K a spectral component was observed whose contribution changed with temperature, yielding the activation energy of about 2 kJ/mol, characteristic for jumps between inversion-related water positions in clusters

Effects of boron addition on the microstructure and properties of in situ synthesis TiC reinforced Cu Ti C composites

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Interplay between local structure and nuclear dynamics in tungstic acid:a neutron scattering study

We provide an exhaustive characterization of structural properties and nuclear dynamics in tungstic acid (WO3·H2O). To this end, we employ neutron and X-ray diffraction (ND and XRD) combined with inelastic neutron scattering (INS) and neutron Compton scattering (NCS) experiments, and we corroborate the analysis with extensive ab initio modeling. The first step in our analysis is the elucidation of the crystal structure based on the refinement of low-temperature powder ND data, extending the knowledge gained from XRD analysis of a mineral specimen of tungstite. These results are confronted with low-temperature INS experiments and zero-temperature phonon calculations. The analysis reveals an inconsistency in the definition of the structure of confined water with respect to crystallographic data, also showing a concomitant failure of the phonon calculations due to a strongly anharmonic confining potential. Extending the computational route toward ab initio MD (AIMD) simulations allows us to probe different structural configurations and provides an improved description of the vibrational dynamics as compared to high-resolution INS experiments, nevertheless, requiring the use of effective classical temperatures. The analysis of both INS and the NCS data reveals a remarkable similarity to the nuclear dynamics earlier reported for water confined in single-wall carbon nanotubes (SWNT), which has been qualitatively described as a new phase of ice. Our analysis reveals a strong two-dimensional hydrogen-bonding network, similar to the shell model for water in SWNT. The reported NCS data show narrowing of the hydrogen momentum distribution with respect to the reference ab initio calculations, indicating a great deal of conformational freedom due to spatial delocalization of protons in the ground state of the system, a clear signature of the quantum character of the nuclei.The authors gratefully acknowledge the financial support from the U.K. Science and Technologies Facilities Council (STFC), granting the beamtime on WISH, TOSCA, and VESUVIO. The authors also acknowledge the computing resources provided by the STFC Scientific Computing Department’s SCARF cluster. K.D. gratefully acknowledges financial support from the Gipuzkoako Foru Aldundia under Grant Number 2020-CIEN-000009-01. This work has been gratefully supported by PL-Grid Infrastructure and the PROMETHEUS facility.Peer reviewe