Anion ordered and ferroelectric ruddlesden-popper oxynitride Ca₃Nb₂N₂O₅ for visible-light-active photocatalysis

A new ferroelectric perovskite oxynitride is proposed and demonstrated for photocatalytic applications using a systematic first-principles study. Ruddlesden-Popper (RP) Ca₃Nb₂N₂O₅, a layered structural derivative of the parent nonpolar perovskite CaNbO₂N, can exhibit a^-a^-c⁺ octahedral-rotation-induced ferroelectricity due to hybrid improper ferroelectricity. We use first-principles calculations to reveal that RP Ca₃Nb₂N₂O₅ exhibits a sizable ferroelectric polarization up to 25 μC/cm² along the in-plane crystallographic direction. As a^-a^-c⁺ octahedral rotations are pervasive within the Ca₃Nb₂N₂O₅ lattice, rotation-induced ferroelectricity is weakly dependent on the anion arrangement and nearly homogeneous throughout the entire configuration space. Furthermore, our electronic structure calculations indicate that ferroelectric Ca₃Nb₂N₂O₅ exhibits a direct band gap of 2.15 eV, strong visible light absorbance up to 580 nm, and dispersive energy bands along the in-plane directions. The spectrally suitable band gap and spontaneous ferroelectric polarization, benefiting the separation of photoexcited electron-hole pairs, enable Ca₃Nb₂N₂O₅ to display promising photocatalytic performance over the visible spectrum. Finally, we demonstrate the prevalence of planar cis-type O/N arrangements in Ca₃Nb₂N₂O₅: the apical anion sites are fully occupied by O and equatorial sites have 1:1 O/N mixed occupancy. Such a robust 1 O/ 2(O_0.5N_0.5) partial anion order should be detectable using standard experimental measurements, making RP Ca₃Nb₂N₂O₅ a unique perovskite oxynitride to investigate the interplay among ferroelectricity, octahedral rotations, and O/N anion order.