Structural chemistry of the n = 3 Dion–Jacobson phases: controlling polarity and band gap†

The Dion–Jacobson phases are increasingly gaining attention for their photoactivity and potential applications as photocatalysts and sensors, in addition to their polar structures. The structure – composition – property relationships for the n = 3 phases (of general formula A′A₂B₃O₁₀e.g., A′ = Rb, Cs; A = Ca, Sr, Ba; B = Nb, Ta) are not well understood because of the lack of reliable structural models for this series. Our combined experimental and computational study addresses this by determining and explaining the complex structural chemistry of these materials, and provides a guide to allow structures and properties of new materials to be predicted and understood. We find that both A′ and A cations determine the tilts of BO₆ octahedra and hence small A²⁺ ions tend to give materials with larger band gaps. The polar structures observed for A = Ca phases result from “proper” mechanisms and the role of the second-order Jahn–Teller effect of the B = Nb, Ta cations is discussed.