Atom relaxations around hydrogen defects in lanthanum hydride

Atom relaxations around empty octahedral and tetrahedral metal interstices in cubic lanthanum hydride LaH2+x have been studied experimentally and by ab initio theory. In-situ neutron powder diffraction data on the deuteride along the pressure-composition isotherm at 573 K reveal deuterium atom displacements of up to 0.6 Å within octahedral interstices. The displacements are directed from the centres towards the faces of the metal octahedra and are largest at deuterium contents of x∼0, i.e. in the presence of nearly empty octahedral and nearly filled tetrahedral interstices. As the deuterium content is increased the displacements decrease sharply and then increase slowly. Ab initio calculations on nearly stoichiometric LaH3 (La32H95) confirm energetically favoured atom relaxations around empty hydrogen sites. The relaxations around tetrahedral vacancies are considerably larger than those around octahedral vacancies. Deuterium atoms in octahedral interstices, for example, relax by up to 0.8 Å which compares well with experiment. Empty metal tetrahedra contract by 0.06 Å while empty octahedra expand by 0.02 Å. Due to the strong relaxations around tetrahedral vacancies, formation of tetrahedral vacancies in LaH3 is less favourable by just about 4 kJ mol−1 than formation of octahedral vacancies.