Structural Studies of the Palladium-Hydrogen System

Abstract

In this work the palladium-hydrogen system has been studied using experimental and computational techniques. The experimental technique used was in-situ neutron powder diffraction. The in-situ methodology, in which diffraction data are collected from a sample loaded to a known hydrogen concentration, provides an independent measure of the hydrogen occupation of interstitial lattice sites. Theoretical modeling with the ADF-BAND software was done to calculate lattice parameters and investigate interstitial occupancy. An important driver for this work was a new report of occupation of tetrahedral interstices near the thermodynamic critical temperature, above which the hydriding phase transformation is continuous. The two-pronged approach was adopted to provide experimental input to the modeling and theoretical understanding of the experiments. The focus of the work carried out was the peri-critical region of the palladium-hydrogen system. While raising significant technical challenges to the experiments, this meant that modeling was made easier, as the system is single-phase above the critical temperature. Fairly direct comparisons of theory and experiment were therefore possible. The experiments performed have revealed much new information about the previously well studied palladium-hydrogen system. Differences have been identified in some thermodynamic properties of this system based upon the bulk form of the palladium, whether solid sheet or finely divided. The two forms have different shapes to their pressure-composition isotherms, the form of the hysteresis is different and they display markedly different thermodynamic critical temperatures. The determination of the thermodynamic critical temperature was also focused on. The classical method of determining critical temperature by the disappearance of the hysteresis in the pressure-composition-temperature (p-c-T) curves has proved to be inaccurate. Above the critical temperature, but still in the peri-critical region, where the hysteresis in the p-c-T curves proves undetectable, it is possible to distinguish two phases via diffraction pattern refinement. The nature of these phases is slightly different to the typical and phases located well below the critical temperature where the phases behave very differently. In the peri-critical region the two phases are very similar and each follows the Vegard relationship independently. A significant part of the research was directed at the location of deuterium atoms in the palladium face centered cubic (FCC) lattice. Two sites in the FCC lattice are available to the deuterium, the tetrahedral site and the octahedral site, and traditionally it is thought that only the octahedral site is occupied. Based on fundamental calculations on peak heights, as well as on full Rietveld refinement, it has been shown that there is now compelling evidence for tetrahedral occupation in the peri-critical region of palladium-deuterium. The theoretical calculations performed accurately estimated the lattice parameters for the range of palladium-hydrogen stoichiometries. By enlarging the modeled unit cell, several hydrogen concentrations could be represented. These calculations provided support for the idea of tetrahedral site occupation by hydrogen of the palladium FCC lattice, and predicted the experimentally observed trend to higher tetrahedral occupancy in the middle of the concentration range Pd to PdH.