Dynamic stability of palladium hydride: An ab initio study (original) (raw)

Vacancy diffusion in palladium hydrides

Physical chemistry chemical physics : PCCP, 2021

The self-diffusion coefficients of palladium in PdHx (x = 0, 0.25, 0.5, 0.75, 1) were studied using density functional theory to obtain the required thermodynamic and kinetic parameters. The enthalpy of migration decreased from 0.95 eV for Pd to 0.78 eV for PdH. The enthalpy of vacancy formation exhibited a substantial decrease from about 1.1 eV in Pd to 0.4 eV in PdH, which was ascribed to successive filling of antibonding states weakening the Pd-Pd bonds. Concurrently, the Arrhenius pre-exponential was significantly reduced from 4.75 × 10-3 cm2 s-1 for Pd to 5.67 × 10-9 cm2 s-1 for PdH due to softening of the vibrational modes that determine the entropy of vacancy formation and initial/transition state frequencies. A linear correlation between the logarithm of the pre-exponential and the activation energy was interpreted as enthalpy-entropy compensation (Meyer-Neldel rule). The Pd self-diffusion coefficients in the hydrides were within 1 order of magnitude of that in pure palladiu...

Electronic structure and crystal phase stability of palladium hydrides

Journal of Applied Physics, 2014

The results of electronic structure calculations for a variety of palladium hydrides are presented. The calculations are based on density functional theory and used different local and semilocal approximations. The thermodynamic stability of all structures as well as the electronic and chemical bonding properties are addressed. For the monohydride, taking into account the zero-point energy is important to identify the octahedral Pd-H arrangement with its larger voids and, hence, softer hydrogen vibrational modes as favorable over the tetrahedral arrangement as found in the zincblende and wurtzite structures. Stabilization of the rocksalt structure is due to strong bonding of the 4d and 1s orbitals, which form a characteristic split-off band separated from the main d-band group. Increased filling of the formerly pure d states of the metal causes strong reduction of the density of states at the Fermi energy, which undermines possible long-range ferromagnetic order otherwise favored by strong magnetovolume effects. For the dihydride, octahedral Pd-H arrangement as realized e.g. in the pyrite structure turns out to be unstable against tetrahedral arrangemnt as found in the fluorite structure. Yet, from both heat of formation and chemical bonding considerations the dihydride turns out to be less favorable than the monohydride. Finally, the vacancy ordered defect phase Pd3H4 follows the general trend of favoring the octahedral arrangement of the rocksalt structure for Pd:H ratios less or equal to one.

Investigation of the vacancy-ordered phases in the Pd–H system

Journal of Alloys and Compounds, 2001

It has been shown that hydrogen-metal reactions operated at high pressures (3-5 GPa) may lead to hydrogen-induced lattice migration. The occurrence of fast diffusion processes that take place within the metal lattice has been established. Under these conditions, modifications of the diffusion kinetics and of the phases equilibria allow to produce vacancy-ordered phases with high vacancy concentrations (20%). An alternative route which leads to such phases that are stable at ambient pressure and temperature is presented. The structural properties of the Pd-(vacancy, H) system which have been studied by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy will be discussed. In the case of palladium, the vacancy-ordered state is characterized by the loss of superconductivity with respect to the Pd hydride. This spectacular modification of the physical properties will be presented and discussed in the light of band structure calculations that have been performed modeling different types of decorated vacancies with octahedral coordination.

Stability of a hydrogen molecule in a vacancy of palladium hydrides

International Journal of Hydrogen Energy, 2012

We report our ab-initio calculations of energy states of equilibrium HeH separation in a vacancy of palladium and palladium hydrides at a variety of H/Pd loading ratios. In a vacancy of pure palladium, the H 2 molecule has a shallow local energy minimum only in the [001] direction at a separation of 0.96 Å and it dissociates into positions near interstitial sites due to its high energy state. Increasing the H/Pd ratio to the beta phase deepens the energy well of the H 2 molecule and results in a shorter HeH separation. At a loading ratio around 1, the H 2 molecule is mostly affected by surrounding hydrogen neighbors and the HeH separation reaches 0.77 Å. The H 2 molecule is then fairly stable and its energy state is comparable to that of nearby interstitial sites. Our calculations suggest that the loading ratio of hydrogen in palladium has a significant effect on the stability of the H 2 molecule in the vacancy.

Stabilization of Lattice Defects in HPT-Deformed Palladium Hydride

Materials Science Forum, 2010

Recent investigations on palladium hydride (Pd-H) showed, for the first time, evidence of formation of vacancy-hydrogen (Vac-H) clusters during Severe Plastic Deformation (SPD) effected by High Pressure Torsion (HPT). Vacancy concentrations produced in Pd-H by this method are extraordinarily high. DSC-scans show that the thermal stability range of vacancies is extended by about 150K due to trapping of hydrogen leading to the formation of vacancy-hydrogen clusters. Recent experiments give evidence that the mobility of the H atoms and/or the vacancies is conditional for the formation of Vac-H clusters during HPT. Results furthermore indicate defect stabilization by hydrogen trapping not only for vacancy-type defects but also for dislocations and grain boundaries.

The structural vacancies in palladium hydride. Phase diagram

International Journal of Hydrogen Energy, 2011

The theory development of structural vacancies formation in palladium hydride on the molecular-kinetic presentations is the subject of this paper. The production of vacantordered superstructure of Cu 3 Au type has been considered at the high temperatures. The calculation of free energies of the PdH and Pd 3 VH phases has been carried out. The constitution diagram defined the temperature and concentration regions of phases formation with the A1 and L1 2 structures and regions of two A1 þ L1 2 phases realization has been constructed. The results of theoretical calculations are in agreement with experimental data.

Density of states of impure palladium deutride/hydride

1996

The dynamical matrix method with fixed force constants is used to calculate the density of states of a palladium deutride/hydride lattice with vacancies. Perturbative and non-perturbative techniques are used to try and speed up the computation. The most important computation is the density of states calculation of a perfect palladium deutride/hydride lattice with one Pd vacancy. It is found that the vacancy modes pile up near the bottom of the optical band.

First-principles study of vacancy-hydrogen interaction in Pd

Physical Review B, 2009

Hydrogen absorption in face-centered-cubic palladium is studied from first principles, with particular focus on interaction between hydrogen atoms and vacancies, formation of hydrogen-vacancy complexes, and multiple hydrogen occupancy of a Pd vacancy. Vacancy formation energy in the presence of hydrogen, hydrogen trapping energy, and vacancy formation volume have been calculated and compared to existing experimental data. We show that a vacancy and hydrogen atoms form stable complexes. Further we have studied the process of hydrogen diffusion into the Pd vacancy. We find the energetically preferable position for hydrogen to reside in the palladium unit cell in the presence of a vacancy. The possibility of the multiple hydrogen occupancy ͑up to six hydrogen atoms͒ of a monovacancy is elucidated. This theoretical finding supports experimental indication of the appearance of superabundant vacancy complexes in palladium in the presence of hydrogen.

Development of physics based analytical interatomic potential for palladium-hydride

Journal of molecular modeling, 2017

Palladium hydrides (Pd-H) research is an important topic in materials research with many practical industrial applications. The complex behavior of the Pd-H alloy system such as phase miscibility gap, however, presents a huge challenge for developing reliable computational models. The embedded atom method (EAM) offers an advantage of computational efficiency and being suited to the metal-hydride system. We propose a new EAM interatomic potential for the complete mathematical modeling of palladium hydride. The present interatomic potential well predicts the lattice constant, cohesive energy, bulk modulus, other elastic constants, and stable alloy crystal structures during molecular dynamics simulations. The phase miscibility gap is also accurately predicted for the Pd-H system using the present potential. To our knowledge, only two Pd-H EAM potentials were used for predicting the phase miscibility gap for the PdH system. The predicted values from these works, however, considerably de...