Ground-state structures, electronic structure, transport properties and optical properties of Ca-based anti-Ruddlesden-Popper phase oxide perovskites

Anti-Ruddlesden-Popper (ARP) phase oxide perovskites ${\mathrm{Ca}}_{4}\mathrm{O}{A}_{2}$ $(A=\mathrm{P}, \mathrm{As}, \mathrm{Sb}, \mathrm{Bi})$ have recently attracted great interest in the field of ferroelectrics and thermoelectrics, whereas their optoelectronic application is limited by their indirect band gaps. In this work, we introduce $A$-site anion ordering in ${\mathrm{Ca}}_{4}\mathrm{O}{A}_{2}$ $(A=\mathrm{P}, \mathrm{As}, \mathrm{Sb}, \mathrm{Bi})$, and find that it induces an indirect-to-direct band gap transition. Using first-principles calculations, we study the ground-state structures, electronic structure, transport properties and optical properties of anion-ordered ARP phase oxide perovskites ${\mathrm{Ca}}_{4}\mathrm{O}A{A}^{\ensuremath{'}}$. Based on analyses of the lattice dynamics, the ground-state structures of ${\mathrm{Ca}}_{4}\mathrm{OAsSb}$ and ${\mathrm{Ca}}_{4}\mathrm{OAsBi}$ are identified in $P4/nmm$ symmetry and those of ${\mathrm{Ca}}_{4}\mathrm{OPSb}$ and ${\mathrm{Ca}}_{4}\mathrm{OPBi}$ are in the $I222$ symmetry. In contrast to the Ruddlesden-Popper (RP) phase oxide and halide counterparts, ${\mathrm{Ca}}_{4}\mathrm{O}A{A}^{\ensuremath{'}}$ $(A{A}^{\ensuremath{'}}=\mathrm{PSb}, \mathrm{PBi}, \mathrm{AsSb}, \mathrm{AsBi})$ show larger band dispersion along the out-of-plane direction, smaller band gaps and highly enhanced out-of-plane mobilities, which results from the short interlayer distances and the enhanced covalency of the pnictides. Although the out-of-plane mobilities of these $n=1$ ARP phase perovskites highly increase, the comparatively strong polar optical phonon scattering limits the further enhancement of their mobilities. Furthermore, compared to RP phase halide ${\mathrm{Cs}}_{2}{\mathrm{PbI}}_{2}{\mathrm{Cl}}_{2}$, ${\mathrm{Ca}}_{4}\mathrm{O}A{A}^{\ensuremath{'}}$ show strong optical absorption around the band edges, and their optical absorption coefficients can reach ${10}^{5}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ within the visible light region due to small band gaps. This study reveals that these ARP phase oxide perovskites exhibit the potential for optoelectronic applications.