The electronic structure of beta-phase palladium hydride

The electronic energy bands for some beta-phase palladium hydrides have been computed using the augmented plane wave method. TheX exchange potential was used. It was found that the localizedd-type states are insensitive, in a relative sense, to the hydrogen concentration, while the other states are constantly elevated in energy as the hydrogen concentration is decreased. Some differences are noticed when the present bands are compared to the photoemission spectra, but the computed Fermi level density of states is in good agreement with electronic specific heat and magnetic susceptibility measurements.Work supported by the Alexander von Humboldt Stiftung.     

Localized vacancies in palladium produced by thermal neutron capture recoil

The environment of recoil atoms in Pd metal after the (n, γ)-process in ¹¹⁰Pd at low temperatures has been studied by perturbed angular correlations of ¹¹¹Cd probes. It shows that 19(1) % of the recoil atoms have a vacancy as nearest neighbour.     

Hydrogen adsorption on palladium and palladium hydride at 1 bar

The dissociative sticking probability for H₂ on Pd films supported on sputtered Highly Ordered Pyrolytic Graphite (HOPG) has been derived from measurements of the rate of the H–D exchange reaction at 1 bar. The sticking probability for H₂, S, is higher on Pd hydride than on Pd (a factor of 1.4 at 140 °C), but the apparent desorption energy derived from S is the same on Pd and Pd hydride within the uncertainty of the experiment. Density Functional Theory (DFT) calculations for the (1 1 1) surfaces of Pd and Pd hydride show that, at a surface H coverage of a full mono layer, H binds less strongly to Pd hydride than to Pd. The activation barrier for desorption at a H coverage of one mono layer is slightly lower on Pd hydride, whereas the activation energy for adsorption is similar on Pd and Pd hydride. It is concluded that the higher sticking probability on Pd hydride is most likely caused by a slightly lower equilibrium coverage of H, which is a consequence of the lower heat of adsorption for H on Pd hydride.     

The energy loss spectra and auger spectra of palladium hydride and palladium glasses

We have measured the Auger and energy loss spectra of Pd hydrides and Pd glasses. An unusually sharp electronic transition was observed at 32.4 eV for pure Pd hydrides. The surface compositions of Pd glasses were found to be in good agreement with their bulk compositions.     

Atomic hydrogen desorption from thin palladium hydride films

It has been proved that hydrogen atoms desorb from the surface of a decomposing thin palladium hydride film. A thin gold film deposited and sintered in situ was used as a selective adsorbent for atomic hydrogen. The TDMS (thermal desorption mass spectrometry) technique was applied to detect the adsorption of hydrogen on gold and to determine the amount adsorbed.     

High pressure X-ray studies of palladium hydride and deuteride

Abstract

The palladium hydride and deuteride have been investigated in the diamond anvil cell up to 35 GPa using energy dispersive x-ray diffraction method. The search for the neutrons in the case of PdD up to the highest pressure reached was unsuccessful.     

Band structure of palladium hydride by the augmented-plane-wave method

The band structure of palladium hydride (PdH) is calculated using the augmented-plane-wave method. Using these calculations the Fermi energy, Fermi surface, de Haas-van Alphen frequencies, and density of states have been determined. These results are compared with our previous calculations for Pd and with other reported results for PdH.     

On the self-consistent statistical theory of vacancies

We report here a new method of describing the effect of the presence of vacancies on the structure, dynamics and thermodynamics of a crystal. The unsymmetrized self-consistent approximation is used to determine the potential energy in which the defective structure and the anharmonicity appear from the beginning. In order to stress the power of this method, we calculate the free energy of formation of a vacancy and the concentration of vacancy in a two-dimensional triangular Lennard-Jones crystal. A comparison with Monte Carlo simulation is given and a possible application to experimentally accessible systems is discussed.     

Implanted muon study of superlattice ordering in palladium hydride PdH0.64

The superlattice ordering transition in PdH_0.64 is detected by implanted muon spectroscopy. The temperature dependence around 50K of the static μSR depolarisation rate, measured in low transverse magnetic field in a polycrystalline sample, indicates appropriate changes in the average number of nearest neighbour protons. These measurements establish the similarity of the proton-proton and muon-proton interactions within the interstitial lattice. The implanted muons reveal the onset of short range order as the transition is approached and, to the extent that vacancy sites are available, participate in the predicted structure below the critical temperature.     

Effect of hydrogen concentration on superconductivity and clustering in palladium hydride

A method of preparing superconducting palladium hybride by electrolysis is described. The relationship of the superconducting transition temperature to hydrogen concentration and the behavior of the hydrogen clustering temperature at high concentrations are reported.     

Ordering of structural vacancies in vanadium monoxide of substoichiometric composition

The order-disorder phase transition in vanadium monoxide of substoichiometric compositions VO_y has been studied for the first time in situ by the Faraday method with a pendulum magnetic balance of the Domenically type. A minimum is found on the temperature dependence of the magnetic susceptibility of vanadium monoxide of different compositions at a temperature of about 700 K. X-ray diffraction analysis performed before and after measuring the magnetic susceptibility confirmed the change in the structure of vanadium monoxide and the possibility of atom-vacancy processes upon cooling and heating of the samples.     

Correlation between band structure and hydride formation in dilute palladium alloys

Data on the free energy change ΔG, following solution of hydrogen in dilute Pd-alloys Pd_1?xM_x have been reviewed for different concentrations of M (M = Au, Ag, Pt, Ir, Rh, V, Cu, Ni, Pb, Sn and Ti) in both the α and β phases. The dependence of ΔG values upon the nature of the substituents (transition metals) is consistently explained within the framework of a metal-hydrogen bonding mechanism in the hydrides. For the β-hydride the ΔG values can be calculated on the basis of the equation ΔG = ΔG_pd + a(T)(〈?^M_LB〉 ? 〈?^Pd_LB〉)x, where $\text{ΔG}{}_{\mathrm{<mtext>Pd</mtext>}}\text{= ? 0.0489}\text{eV}\text{H}$ atom and is the free energy change of solution of hydrogen in pure Pd, a(T) = 0.194 at T = 298 K, 〈?^m_LB〉 and 〈?^pd_LB〉 are the average energies of the lowest band of the pure constituents ($\text{〈?}{}^{\mathrm{<mtext>Pd</mtext>}}{}_{\mathrm{<mtext>LB</mtext>}}\text{〉 = ?9.15}\text{eV}\text{atom}$). The stability of the palladium-hydrogen bond in dilute Pd-alloys depends on the value of 〈?^M_LB〉; for substituents having lower 〈?^M_LB〉 values than Pd the bond will strengthen, while for those having higher 〈?^M_LB〉 values it will weaken. This behaviour agrees well with the general trend of the stability of the stoichiometric hydrides predicted by Gelatt, Ehrenreich and Weiss using band structure results.     

Investigation of structural vacancies in titanium monoxide by electron-positron annihilation

Local atomic environment of vacancies in nonstoichiometric titanium monoxide ranging in composition from TiO_0.74 to TiO_1.26 was studied by electron-positron annihilation. Analysis of the Doppler broadening spectra of the annihilation gamma line for titanium and liquid oxygen showed that positrons in titanium monoxide are trapped by titanium vacancies. Experiments revealed that the lifetime of positrons in ordered and disordered titanium monoxide TiO _y increases with increasing oxygen content y and varies from 184 to 210 ps. Data on the valence electron density permitted the prediction that the lifetime of free positrons in stoichiometric titanium monoxide is about 140 ps and the lifetime of positrons localized in an oxygen vacancy is about 170 ps. The method used to analyze the gamma-line Doppler broadening spectra makes it possible to determine the type and number of atoms around a vacancy and to investigate order-disorder phase transformations in nonstoichiometric compounds.     

Multiple scattering theory of diffuse LEED from vacancies

Previous work has shown how to make a full multiple scattering theory of DLEED from isolated adatoms. Here we investigate the complementary problem of scattering from isolated vacancies. Implementation of the theory involves two conventional LEED calculations, for the incident and exit beams and a cluster calculation, similar to that used in EXAFS, centred on the vacancy.     

Observation of structural vacancies in titanium monoxide using transmission electron microscopy

Structural vacancies were directly observed in nonstoichiometric ordered titanium monoxide using high-resolution transmission electron microscopy under a magnification of 4×10⁶. The observation of structural vacancies became possible due to their ordering and the formation of continuous vacancy channels in certain crystallographic directions. Microdiffraction was employed to orient the sample in the direction permitting the observation of vacancy channels. Transmission electron microscopy providing a magnification of tens of thousands of times revealed that titanium monoxide grains do not contain cracks and macropores and confirmed that the free volume detected picnometrically in the titanium monoxide is concentrated in structural vacancies on the titanium and oxygen sublattices.     

Tunneling observation of the superconducting energy gap in the palladium hydride thin films

Superconductivity in hydrogen charged palladium thin films has been studied. Using niobium based tunneling junctions the energy gap in PdH_x samples, with different values of x and hence of T_c have been observed.     

Strain relief and Pd island shape evolution on the palladium and palladium hydride (100) surface

The mesoscopic relaxation of small Pd islands on Pd(100) and PdH(100) surfaces is investigated on the atomic scale by performing molecular statics calculations. A strong strain and stress inhomogeneity in islands and topmost layers of the substrate is revealed. An unusual size dependence of the shape of islands is discovered.