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(O0.5N0.5) 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.