title: Hydrogen Properties in Palladium
Creators: name: Setayandeh, Samaneh S
Publisher: name: Griffith University
Year: 2021
Subjects: subject: palladium
subject: hydrogen
subject: palladium-hydrogen system

description: In this project the palladium-hydrogen system was investigated, using computational and experimental approaches. Computational modelling, employing ab initio calculations based on density functional theory (DFT), were done with the Quantum Espresso package, to investigate the electronic and phonon properties of PdH and its superabundant vacancy phase (Pd3VacH4), assuming that octahedral or tetrahedral lattice interstices are occupied by hydrogen, and to relate these to important physical properties, in particular the atomic volume of hydrogen in Pd and the superconducting transition temperature of PdH. The driver for the computational part of this project was the history of DFTbased publications in the literature predicting such widely different phonon band structures that confident interpretations of the results really could not be made. Therefore, a systematic approach was followed to compare the calculated electron and phonon properties of PdH and Pd3H4 with six different DFT schemes, within the harmonic approximation. The results highlighted the excellent agreement for the electron band structure and the sensitivity of the phonon properties to the details of the schemes employed, and also confirmed the need to include anharmonicity of the H potential to obtain realistic phonon results. The calculations predicted dynamic instability for octahedral Pd3VacH4, although the calculated lattice constant agreed with the estimated zerotemperature experimental value. This structure requires new calculations accounting for anharmonicity. The calculated lattice constant for Pd3VacH4 in the tetrahedral case was larger than any experimental value, so this alternative, while dynamically stable, is certainly not observed. While the absolute lattice constant of PdH varied considerably between DFT schemes, it was found that the partial atomic volume of H in Pd, a relative measure, was rather robust against differing computational approaches. A significant discrepancy was demonstrated between the average DFT predictions for the atomic volume assuming octahedral-only interstitial occupancy and that calculated from neutron diffraction measurements on Pd and PdHx. Calculations for PdH with mixed octahedral and tetrahedral occupancy showed that PdH containing around 20% tetrahedral H would have the correct atomic H volume. Within the harmonic approximation, the calculated superconducting transition temperature of octahedral PdH varied by a factor of nearly three across the six DFT schemes employed. This shows that a reliable DFT approach is needed before attempting to account for anharmonicity of the H potential. In the tetrahedral case, less discrepancy was observed between the characteristic measures of superconductivity, due to the smaller contribution of high-frequency optical branches to the electron-phonon coupling parameter, but the actual values predicted for the transition temperature should be relatively reliable because the tetrahedral site, being relatively small, should be more nearly harmonic than the octahedral site. The transition temperatures calculated for tetrahedral PdH were higher than those measured experimentally, making partial tetrahedral occupancy an interesting possibility from the superconductivity point of view. For the experimental part of this project, high-resolution gravimetry was used to survey the pressure-composition behaviour of PdHx and PdDx formed above their critical points and cooled to room temperature, to determine if this preparation route leads to extra hydrogen and deuterium absorption compared to passage through the two-phase region. It was found that supercritical absorption shifted the low-pressure portion of the desorption isotherm towards higher concentration. However, the isotherms were shifted towards the expected H/D concentration by cycling, apparently due to introducing misfit dislocations in the sample. Hydriding the sample at room temperature and heating it to 300 ℃ under pressure instead of first heating to 300 ℃ and then loading with hydrogen unexpectedly shifted the supercritcal isotherms at 300℃ to lower pressure, pointing to a further and previously unobserved influence of defects on the absorption-desorption behaviour.