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.

中文翻译：

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可见光活性光催化的阴离子有序铁电Ruddlesden-Pop氧氮化物Ca ₃ Nb ₂ N ₂ O ₅

提出了一种新的铁电钙钛矿氧氮化物，并通过系统的第一性原理研究证明其可用于光催化应用。Ruddlesden-波普（RP）的Ca ₃的Nb ₂ Ñ ₂ ø ₅，一层叠在母非极性钙钛矿CANBO的结构衍生物₂ N，可表现出^-一个^- ç ⁺八面体旋转引起的铁电由于混合不当铁电性。我们使用第一性原理计算得出，RP Ca ₃ Nb ₂ N ₂ O ₅表现出高达25μC/ cm ²的相当大的铁电极化沿面内结晶方向。由于在Ca ₃ Nb ₂ N ₂ O ₅晶格中普遍存在a ^– a ^– c ⁺八面体旋转，旋转引起的铁电微弱地依赖于阴离子排列，并且在整个配置空间中几乎是均匀的。此外，我们的电子结构计算表明铁电Ca ₃ Nb ₂ N ₂ O ₅表现出2.15 eV的直接带隙，高达580 nm的强可见光吸收率以及沿面内方向的分散能带。光谱合适的带隙和自发的铁电极化，有利于光激发电子-空穴对的分离，使Ca ₃ Nb ₂ N ₂ O ₅在可见光谱上显示出有希望的光催化性能。最后，我们证明了Ca ₃ Nb ₂ N ₂ O ₅中平面顺式O / N排列的普遍性：顶端阴离子位点被O完全占据，赤道位点的O / N混合比为1：1。如此强大的1 O / 2（O _0.5N _0.5）部分阴离子级应使用标准实验测量值检测，使RP Ca ₃ Nb ₂ N ₂ O ₅成为独特的钙钛矿型氧氮化物，以研究铁电，八面体旋转和O / N阴离子级之间的相互作用。