Abstract
The structural and electronic properties of pure and hydrogenated Li metal have been calculated by modelling the crystalline solid in terms of small clusters of constituent atoms. Using the self-consistent molecular orbital method in the Hartree-Fock approximation, the authors have investigated the minimum number of atoms necessary to distinguish among various crystalline phases. They find that with as few as three atoms in the molecular cluster, the BCC phase can be shown to be the most stable structure. They have calculated the dependence of the optimised lattice parameter as a function of cluster size for both FCC and BCC phases of Li. While the calculated lattice constants converge to the bulk value with only about 20 atoms in the cluster, the binding energy per atom shows no sign of saturation. Thus how well the properties of molecular clusters represent bulk characteristics depends on the properties being investigated. This aspect is further illustrated by calculating the equilibrium site of hydrogen in Li. It is shown that for small clusters consisting of less than 20 atoms, the preferential site of hydrogen depends not only on the cluster size, but also on the way the cluster is constructed. The distortion of the nearest neighbour atoms around the hydrogen atom is studied by minimising the total energy with respect to the nearest-neighbour Li-hydrogen distance. Contrary to the conventional wisdom, they find that the metal atoms surrounding hydrogen are not always displaced outward. For example, the Li atoms around an octahedral hydrogen atom relax outward, while those around a tetrahedral hydrogen atom relax inward. In both cases, the equilibrium distance between hydrogen and nearest Li atom is 3.48 au. This is comparable with the corresponding distance of 3.6 au in stoichiometric lithium hydride. They have also calculated the charge transfer between Li and hydrogen atoms and the population of various orbital states for all the Li atoms. The results are used to analyse the screening of impurities in the metallic environment and to shed light on Mossbauer isomer shifts in metal-hydrogen systems.