Perovskite-type oxyhydride with a two-dimensional electron system: First-principles prediction of KTiO₂H

We investigate an unsynthesized perovskite-type oxyhydride $KTiO2H$ using first-principles calculations. Based on a comparison of the total energy with some other configurations of hydrogen and non-perovskite-type structures, the presented structure is found to be more stable. The optimized structure of $KTiO2H$ is orthorhombic and its polarization of $101 μC/cm2$ is comparable to that of $PbTiO3$⁠. The density of states is similar to that of the other perovskite-type oxides, except that it is constant near the top of the valence band. At the top of the valence band, the band dispersion is small in the direction perpendicular to the $Ti$–$O$–$H$ plane, which is reflective of the two-dimensionality of the electronic state. Furthermore, the electronic structure is compared with that for $KTiO2F$⁠, and it is found that the dispersionless states can be regarded as antibonding states of in-plane oxygen and hydrogen and that the low electron affinity of hydrogen is important to prevent overlap with other states.