Pd Lattice Research Articles

Thermodynamic Studies of the Absorption of Hydrogen by Pd – Ti(Zr) Solid Solution Alloys

AbstractThe solubility of hydrogen in Pd – Ti and Pd – Zr solid solution alloys containing up to 6.0 at.% Ti and Zr has been measured at temperatures between 273 K and 433 K and hydrogen pressures up to 1000 Torr. The relative partial molar enthalpy, ΔH0H, at infinite dilution for Pd – Ti alloys shows a minor minimum around 4 at.% Ti, in spite of the Pd lattice contraction on Ti substitution. For Pd – Zr alloys the ΔH0H values increase in exothermicity regularly with Zr content. The relative partial molar entropy, ΔS0H, at infinite dilution in both alloy series decreases with the solute contents. The apparent H‐H attractive interactions, g1, in Pd – Ti(Zr) alloys decrease with alloying metal content, and the trend of the g1 value with Ti content is dissimilar to those found for other lattice contracted alloys. The stability of the β‐phase hydride in Pd – Ti alloys decreases with Ti content and the β‐phase hydride in Pd – Zr alloys also becomes less stable with Zr content, albeit by a small amount, despite being a lattice expanded alloy system. Solvus compositions at the αmax phase boundary in both Pd – Ti and Pd – Zr alloys are almost equal at the same alloying compositions of each alloy series, although increasing with alloying metal content in both cases.

Photoemission studies of Pd/D system with high deuterium content

The ultraviolet photoelectron spectroscopy (UPS) spectra of Pd/D system (including absorption at high pressure) with various D contents were measured. The D absorption induced a new peak centered at about 6eV below Ef, which seems to derive from the hybridized states of deuterium and Pd electronic states. Its intensity increases with D content; only part of the D atoms in the Pd lattice are ionized at high D content.

Electron microscopy study of the interaction of Ni, Pd and Pt with carbon: II. Interaction of palladium with amorphous carbon

A change in the lattice parameter of Pd following the evaporation of the metal onto a carbon support is interpreted by the incorporation of carbon atoms into the Pd lattice. The decomposition of this phase above 700 K leads to the deposition of carbon on top of the Pd crystallites or even to their encapsulation. At temperatures above 870 K. larger Pd crystallites were found to catalyze the graphitization of amorphous carbon.

Thermodynamic Behavior of Hydrogen in Palladium‐Gadolinium Solid Solution Alloys

AbstractThe thermodynamic quantities for absorption of hydrogen by disordered Pd — Gd solid solution alloys containing up to 8.0 at.% Gd have been determined at temperatures between 273 and 473 K and hydrogen pressures up to 100 Torr by measuring the pressure‐composition isotherms. The standard relative partial molar enthalpy of solution of hydrogen, ΔH0H, at infinite dilution becomes more exothermic with increasing Gd content. Whereas the standard relative partial entropy, ΔS0H, at infinite dilution decreases with Gd content; this implies that the octahedral interstices are excluded for hydrogen occupation by Gd neighbors. Substitution of Gd in the Pd lattice decreases the apparent H‐H attractive interaction and for the Pd‐4.5 and 6.0 at.% Gd alloys the H‐H interaction becomes repulsive over a certain range of H contents. The results are discussed in terms of the quasi‐chemical model by taking into the constant volume correction. The stability of P‐hydride phase increases with Gd content, and the extent of the phase boundary, i.e., the hydrogen solvus composition at a given temperature, is more favourable by Pd alloying with Gd.

ISING MODEL FOR INTERSTITIAL ALLOYS AND SUPERCONDUCTING OXIDES

The interstitial alloys MZx with Z= H, C, N, O etc. at octahedral or tetrahedral sites of the fcc or bcc M lattice are analysed for nearest and next-nearest neighbor Z-Z interactions. Many compounds have repulsive interactions close to the borderline of maximum separation, which is characterized by Pauling’s electrovalence rule. There are also borderlines for attractive interactions, e.g. cluster formation or segregation. The new superconducting oxides Ba 2 YCu 3 O x, La 2 CuO 4, Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 can be viewed as bcc M= Ba, Cu, Bi etc. lattice with Z= O atoms at octahedral sites. The low T c compounds Nbo , NbC x, TiO x, TiN x, PdH x, etc. have Z= C, N, O, H atoms at octahedral sites of the fcc M= Nb, Ti, Pd lattice.

Effect of high-temperature reduction on carburization of alumina-supported palladium: Evidence for palladium-aluminium alloy formation

The effect of high-temperature treatment in hydrogen on the carburization of palladium in a commercial 10% Pd/Al 2O 3 catalyst was studied by X-ray powder diffraction. It was found that reduction at 970 and 1070 K for 40 h causes contraction of the Pd lattice by 0.22 and 0.38%, respectively, presumably owing to Pd 1− x Al x alloy formation ( x=0.06 at 970 K and 0.10 at 1070 K). As a consequence of the alloying, alumina-supported Pd loses its ability to constitute a supersaturated Pd C solid solution on carburization in acetylene at 550 K. The initial activity of Pd can, however, be restored by a two-step process consisting in oxidation at 770 K followed by reduction in hydrogen at 720 K.

Two innocent chemists look at cold fusion

The authors propose that the large energy release reported in the experiments of Fleischmann, Pons, and Hawkins may be the consequence of {sup 2}H fusion accelerated through screening by neighboring heavy electrons with mass m* {approx} 10 electron masses. The presence of the Pd lattice may also accelerate the radiationless relaxation (RR) of the transient excited {sup 4}He*, perhaps by an internal-conversion-like process. If RR exceeds the rate of fragmentation of {sup 4}He*, this can explain why the bulk of the energy is released as heat rather than via neutron and tritium production. Symmetry considerations show that low-energy fusion can lead to different product channel branching ratios than are observed in high-energy experiments and may allow RR rates to outstrip fragmentation rates. Our analysis also suggests that the rate of {sup 1}H + {sup 2}H fusion may be comparable to or even in excess of that of {sup 2}H + {sup 2}H fusion in the Pd lattice, so that fusion might even be observed in water of natural abundance deuterium content.

Helium, tritium, and protium NMR studies of tritide-free aged PdTx.

Pulsed NMR relaxation-time measurements have been performed on ${\mathrm{PdT}}_{\mathrm{x}}$, aged approximately eight years, after first removing most of the tritium by room-temperature desorption and again after addition of protium to one of these tritide-free samples. Spin-lattice and spin-spin relaxation times ${T}_{1}$ and ${T}_{2}$ were measured for both tritium and $^{3}\mathrm{He}$ in the temperature range 100--300 K, while ${T}_{1}$, ${T}_{2}$, and the rotating-frame relaxation time ${T}_{1\ensuremath{\rho}}$ were obtained for protium in the same temperature range. The relative magnitudes of ${T}_{1}$, the decay time ${T}_{2m}$ of the Carr-Purcell-Meiboom-Gill signals, and the line-shape decay time ${T}_{2}^{\mathrm{*}}$ for $^{3}\mathrm{He}$ in tritide-free ${\mathrm{PdT}}_{\mathrm{x}}$, are consistent with the distribution of the helium as microbubbles in the Pd matrix. The increase in the $^{3}\mathrm{He}$ values of ${T}_{1}$ upon addition of protium to an atomic ratio [H]/[Pd]=0.68 is attributed partially to a decrease in the Fermi-level density of states and partially to other internal relaxation mechanisms. Analysis of the dipolar component ${T}_{1d}$ of ${T}_{1}$, for protons in the sample to which protium was added, gives a value of the diffusion activation energy at temperatures between 230 and 300 K which is in agreement with that for \ensuremath{\beta}-${\mathrm{PdH}}_{\mathrm{x}}$. Comparison with the triton NMR in tritide-free ${\mathrm{PdT}}_{\mathrm{x}}$ indicates that the residual tritium, rather than forming a dilute solid solution, occupies unusual sites in the Pd lattice, perhaps at internal surfaces surrounding the $^{3}\mathrm{He}$ microbubbles.

Structural and magnetic properties of Au/Pd/Au sandwiches: A total energy all-electron approach

The possibility of magnetism induced in Pd when sandwiched between Au layers, first proposed and studied experimentally by Brodsky and Freeman, is investigated by means of highly precise all-electron total energy local spin-density calculations employing the thin-film full-potential linearized augmented plane wave (FLAPW) method. Total energy local density calculations are employed on a Au/Pd/Au sandwich with three Pd layers to obtain the structural properties and five Pd layers to obtain the magnetic properties of an Au/Pd/Au sandwich. Detailed results are obtained with spin polarized calculations for the Pd lattice constant stretched taking into account (i) the misfit between the lattice constants of Au and Pd and (ii) proximity effects in a self-consistent way. The magnetic moment of the center Pd layer is found to be 0.02μB which is consistent with the experiment of Brodsky. A negligible magnetic moment for the interface Pd layers is obtained.

Effects of tunnelling in proton magnetic resonance transient responses in β-phase palladium hydride

A comprehensive study of the effects of hydrogen atom tunnelling on proton magnetic resonance transient responses is reported for beta -phase palladium hydride. Data for beta -PdHx are presented for concentrations X=0.80, 0.75 and 0.71 at 40K and are characteristic of data taken over the temperature range 30 to 80K. It is shown that the free-induction decay signal can be represented by the same empirical function for each concentration studied. The ratio of second to fourth moments deduced by fitting this function to the data is consistent with a random distribution of hydrogen atoms over the octahedral sites of the Pd lattice. However, the moments themselves are reduced from their rigid lattice values as a result of the tunnelling of the hydrogen atoms. A solid-echo study shows that the direct dipolar coupling between protons is much more important than indirect coupling. The presence of a Hahn echo is related to the distribution of Knight shifts in this nonstoichiometric alloy. The characteristics of this echo are documented.

Local hydrogen arrangement around the μ+ in a PdH0.74 single crystalin a PdH0.74 single crystal

The orientation dependence of the inhomogeneously broadened μSR line width has been measured in a single crystal α′-PdH0.74 sample between 20 and 150 K. Protons and muon are thought to occupy octahedral interstitial sites in the Pd lattice. A good fit to the data is only obtained assuming a decrease in the proton occupancy of the octahedral sites nearest to the μ+ with increasing temperature. The proton occupancy of the remaining octahedral sites at all temperatures as well as the occupancy of the proton sites nearest to the μ+ at 20 K is compatible with the bulk nominal hydrogen concentration x=0.74.

Quenching and electron-irradiation effects in ordered beta -PdH(D)0.63 around the resistivity anomaly near 50 K.

\ensuremath{\beta}-PdH(D${)}_{0.63}$ specimens had been quenched across the 50-K anomaly and/or bombarded with electrons of 0.35--0.8 MeV energy at 20 K. They exhibited after each treatment a residual-resistivity decrease, -\ensuremath{\Delta}\ensuremath{\rho}, which annealed upon heating in the anomaly region. A kinetic analysis of the recovery process yielded an activation energy for H-defect migration of ${E}_{m}$(H,D)=50\ifmmode\pm\else\textpm\fi{}15 meV for a first-order reaction. An investigation of the isotope-dependent radiation-induced -\ensuremath{\Delta}${\ensuremath{\rho}}_{\mathrm{irr}}$ as a function of the electron energy led, by fitting theoretical collision cross sections, to a displacement threshold assimilated with a binding energy of hydrogen in its sublattice, ${E}_{b}$\ensuremath{\sim}0.01 eV, and to a defect resistivity, ${\ensuremath{\rho}}_{\mathrm{def}{}^{\mathrm{H},\mathrm{D}=\mathrm{\ensuremath{-}}6}}$ \ensuremath{\mu}\ensuremath{\Omega} cm/unit concentration. The proposed defect configuration is occupational disorder in the hydrogen-vacancy sublattice, created by site interchange of H atoms with vacant octahedral sites of the Pd lattice. This leads to an effective redistribution of the ${\mathrm{Ni}}_{\mathrm{n}}$Mo (n=1--4) type microdomains (Ni equivalent to H, Mo equivalent to H vacancy) as observed by neutron scattering in the anomaly region, by locally forming ${\mathrm{Ni}}_{\mathrm{n}\ifmmode\pm\else\textpm\fi{}1}$Mo-type cells of different symmetry.

Exafs study of the sructure of boron in palladium

Abstract The local lattice structure around B atoms in Pd was determined from the EXAFS at the K-edge of' Pd in to Pd B alloys with B/Pd = 16 and 11.4 %. It was confirmed that B occupies octahedral interstices in the fcc lattice of Pd. The lattice expansion of the Pd matrix by the dissolved B is da/a = (0.169±0.003) for B/Pd = 1. Boron displaces its 6 nearest Pd neighbors outwards by (0.106 ± 0.005)A relative to pure Pd and by (0.061 ± 0.005)A relative to the average Pd B lattice. The local lattice distortion has practically vanished beyond the second Pd neighbors.

The growth mode of Cu overlayers on Pd(100)

The growth mode of Cu deposits on clean Pd(100) has been studied by means of LEED. AES and angular resolved ultraviolet photoemission spectroscopy. Auger signal versus deposition time curves showed that the growth follows a layer-by-layer mode for two monolayers after which it involves formation of islands as in the Stranski-Krastanov model. Despite the large lattice mismatch between Cu and Pd (7.7%) the substrate p(1×1) LEED pattern persisted with clear distinct spots at least up to the equivalent coverage of 4 ML indicating that Cu starts to grow epitaxially with the Pd lattice constant (pseudomorphic growth). Photoemission results show that the spectrum of the Cu/Pd(100) system does not converge to that of bulk Cu(100) until about 65 ML equivalent coverage. The spectrum below that coverage is composed of separate contributions from the thick regions and from the pseudomorphic basic slab.

The growth mode of Cu overlayers on Pd(100)

The growth mode of Cu deposits on clean Pd(100) has been studied by means of LEED. AES and angular resolved ultraviolet photoemission spectroscopy. Auger signal versus deposition time curves showed that the growth follows a layer-by-layer mode for two monolayers after which it involves formation of islands as in the Stranski-Krastanov model. Despite the large lattice mismatch between Cu and Pd (7.7%) the substrate p(1×1) LEED pattern persisted with clear distinct spots at least up to the equivalent coverage of 4 ML indicating that Cu starts to grow epitaxially with the Pd lattice constant (pseudomorphic growth). Photoemission results show that the spectrum of the Cu/Pd(100) system does not converge to that of bulk Cu(100) until about 65 ML equivalent coverage. The spectrum below that coverage is composed of separate contributions from the thick regions and from the pseudomorphic basic slab.

In situ transmission electron microscope studies of palladium on MgO

Palladium particles were grown from the vapor phase inside a controlled-vacuum specimen chamber of a transmission electron microscope (TEM) on freshly prepared and flat MgO substrate surfaces. Annealing and various effects of gas exposure of the particulate Pd deposits were studied in situ by high-resolution TEM and electron diffraction. Even at substrate temperatures of 300 °K (room temperature), the deposits were perfectly epitaxial. A lattice expansion of 2–4% was noted; the highest values of expansion (compared to the bulk Pd lattice parameter) were found for the smallest particles (1–1.5 nm size range). Long-time RT exposure of Pd MgO in a vacuum of 10 −8 mbar (major residual gas components were water and nitrogen) yielded negligible particle mobility and coalescence. However, exposure to laboratory air or oxygen at RT greatly enhanced the particle mobility and coalescence and also resulted in flattening of the deposit particles. Electronbeam irradiation further enhanced this effect. Exposure to laboratory air for several tens of hours led to strong coalescence with a tendency of minimization of the particle surface areas by promoting three-dimensional Pd particle habit formation. The results, therefore, indicate that small deposited metal particles may undergo appreciable changes when they are exposed to air at RT, such as is common practice in ex situ TEM investigations of particulate deposits.

Lattice parameter and57Fe isomer shift in PdHx: evidence for two new phases

Pd lattice parameter and 57Fe isomer shift (IS) measurements have been correlated in PdHx-Fe matrices (0.3 at.% Fe) over a wide range of hydrogen concentration in the alpha ' (lattice liquid), and in the beta and alpha phases detected previously by Mossbauer measurements. The authors find that the IS is proportional to the logarithm of the unit cell volume, IS varies as 3 ln a0, just as in IS measurements under pressure. The experimental value of the slope, +3.92 mm s-1, is however much larger than that found by pressure measurements in similar matrices. This shows that the effect in PdHx is of electronic origin, and gives support to the band calculations which have predicted partial localisation of hydrogen electrons in a Pd-H bond.

Diffusion-induced reduction of the57Fe Mossbauer fraction in βPdHx

The temperature dependence of the 57Fe Mossbauer f factor, center shift and linewidth of 57Co:PdHx sources with x=0 and 0.67 and of 57Fe:PdHx absorbers with x=0.55 and 0.71 were measured between 100 and 300K. In all hydrides an anomalous decrease of the f factor was observed between 200 and 260K. This can be attributed to the transient strains produced by the hydrogen atoms diffusing in the Pd lattice. In the samples with x=0.67 and 0.71, the temperature dependence of the centre shift deviates from the mere second-order Doppler behavior. This reveals changes in the mean population of the hydrogen sites near the Mossbauer probe. The temperature dependence of the linewidth can be understood qualitatively as a superposition of two effects, namely broadening due to the phase incoherence that arises from the fluctuating strains and motional averaging of the hyperfine interactions, which cause an inhomogeneous line broadening at low temperatures.

Nonrigid Lattice Model of Hydrogen Diffusion in a Transition Metal: A Statistical Mechanical Approach

Abstract The isotopes of hydrogen diffuse through transition metals such as Pd with different rates. The observed rates show a complicated temperature dependence including a reversed isotope effect. The diffusion generally behaves nonclassically. Also at moderate densities of hydrogen in Pd, the embedded hydrogen can undergo second-order (mean-field type) phase transition. The transition of this kind implies that there exist long-range attractive forces between H atoms mediated by the Pd lattice. These forces are expected to play a fundamental role in the quantum diffusion of a hydrogen-dense Pd system. To provide a first-principles description of hydrogen diffusion in Pd, we have constructed a physical model based on two ideas: (i) H atoms occupy the interstitial sites of the Pd lattice. (ii) The Pd lattice is nonrigid. To calculate the time correlation function (from which the diffusion coefficient may be deduced), we must for technical reasons remove the nonrigidity from the model via a unitary transfo...

Effect of anharmonicity on superconducting metal-hydrogen systems

An estimate of the effect of anharmonicity on the superconducting transition temperature of stoichiometric palladium-hydride (-deutride) systems has been made. It is found that the static phonon frequency shift which modifies the electron-phonon coupling parameter is mainly due to quartic anharmonicity. The calculated transition temperature of PdD is in close agreement with experiments. This confirms that the inverse isotope effect in PdH (PdD) systems is due to anharmonicity of H (D) atoms in the Pd lattice.