For example, MAPbI₃ crystallizes in a tetragonal perovskite structure at room temperature, consisting of PbI₆ octahedra sharing corners, with MA⁺ cations occupying the interstitial spaces. Variants of the perovskite structure also exist: for instance, layered (2D) perovskites have alternating organic and inorganic sheets (formula A₂BX₄), and “double” perovskites (e.g. A₂BB′X₆) replace Pb²⁺ by a combination of a monovalent and trivalent cation to eliminate lead. However, the defining feature of halide perovskites is the 3D corner-sharing BX₆ framework with monovalent A ions in the cavities

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