On hydrogen point defects in perovskite oxides

Licentiate thesis, 2017

Oxides based on the perovskite structure exhibit a surprisingly large diversity in materials properties and are found in many different applications, several related to clean energy technologies, such as solar cells, batteries and fuel cells. Many properties in materials are the result of lattice imperfections, commonly denoted \emph{defects}, and much effort is devoted to fine tuning materials properties through controlling the defects therein. Therefore, a thorough understanding of defect properties on a microscopic scale is desirable, and first-principles calculations have proven an invaluable tool in complementing experimental observations. In the present thesis density functional theory (DFT) calculations have been employed to describe two types of hydrogen point defects in perovskite oxides with the aim of deepening the understanding as well as to develop tools for modelling and characterising point defects. In paper I a strain tensor formalism for describing the anisotropic volume expansion of a point defect is developed. The formalism is successfully applied to the proton forming a hydroxide ion and the oxygen vacancy in acceptor-doped barium zirconate. It is inferred that both the hydroxide ion and the oxygen vacancy are smaller than the oxygen host ion, but that the difference in size causes an expansion in hydration which could lead to micro-cracking of the material. In paper II the substitutional hydride ion on an oxygen site in barium titanate is investigated. For this oxyhydride material two possible electronic states are permissible leading to different conductive properties; on the one hand the delocalised band-state as predicted by band theory and on the other hand a polaron state, in which an electron localises on one of the titanium ions next to the hydride ion, the description of which requires beyond DFT-methods. The two electronic states are investigated through their influence on the hydrogen vibrations, using both theoretical methods and inelastic neutron scattering measurements, and through their different volume expansion. The conclusion that the electronic state is predominantly band-like is confirmed both through the vibrational characterisation and the strain tensor formalism. The thesis reiterates the usefulness of first-principles calculations in assisting interpretation of experimental data.