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 ${\mathrm{Ca}}_4\mathrm{O}A{A}^{\prime }$. 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, ${\mathrm{Ca}}_4\mathrm{O}A{A}^{\prime }$ $(A{A}^{\prime }=\mathrm{PSb}, \mathrm{PBi}, \mathrm{AsSb}, \mathrm{AsBi})$ 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 ${\mathrm{Cs}}_2{\mathrm{PbI}}_2{\mathrm{Cl}}_2$, ${\mathrm{Ca}}_4\mathrm{O}A{A}^{\prime }$ show strong optical absorption around the band edges, and their optical absorption coefficients can reach ${10}^5\mathrm{c}{\mathrm{m}}^{-1}$ 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.

Figure 1

(a) Structures of ${\mathrm{Ca}}_4\mathrm{O}{A}_2$ aristotype with a single anion occupying the $A$-site and ${\mathrm{Ca}}_4\mathrm{O}A{A}^{\prime }$ with anion ordering arrangements along the [001] direction in the ARP phase. (b) Energetics of ${\mathrm{Ca}}_4\mathrm{O}A{A}^{\prime }$ ($A{A}^{\prime }=\mathrm{PAs}$, PSb, AsSb, PBi, AsBi, SbBi) in $A_1$, $A_2$, and $A_3$ anion ordering. The energies of each composition in $A_2$ configuration are taken as a reference.

Figure 2

Phonon spectra of four A-site anion-ordered ARP phase perovskites ${\mathrm{Ca}}_4\mathrm{O}A{A}^{\prime }$ (a)–(d): ${\mathrm{Ca}}_4\mathrm{OAsSb}$, ${\mathrm{Ca}}_4\mathrm{OAsBi}$, ${\mathrm{Ca}}_4\mathrm{OPSb}$, and ${\mathrm{Ca}}_4\mathrm{OPBi}$.

Figure 3

Band structures and density of states of (a) ${\mathrm{Ca}}_4\mathrm{OAsSb}$, (b) ${\mathrm{Ca}}_4\mathrm{OAsBi}$, (c) DT-${\mathrm{Ca}}_4\mathrm{OPSb}$, and (d) DT-${\mathrm{Ca}}_4\mathrm{OPBi}$ calculated at the $\mathrm{HSE}+\mathrm{SOC}$ level.

Figure 4

Optical absorption spectra and SLME as a function of film thickness of four anion-ordered ARP phase perovskites: (a),(b) in-plane and out-of-plane absorption spectra; (c),(d) in-plane and out-of-plane SLME. The optical absorption and SLME of cubic ${\mathrm{CsPbI}}_3$ (α-phase) and ${\mathrm{Cs}}_2{\mathrm{PbI}}_2{\mathrm{Cl}}_2$ are shown as a reference.