Structures and band gaps of lead-free dabconium-containing hybrid alkali-metal halide perovskites†

Ten hybrid metal halide perovskites containing divalent dabconium cations and alkali-metal ions have been structurally characterised and their band gaps determined, with six new crystal structures reported. Five compounds of the formula (C₆H₁₄N₂)[BX₃], where B = Na⁺ and X = Cl⁻ or Br⁻, B = K⁺ and X = Cl⁻ or Br⁻ and B = Cs⁺ and X = I⁻, were found to exhibit one-dimensional (1D) hexagonal perovskite structures containing face-sharing metal halide octahedra that form 1D inorganic polymers. Five compounds of the formula (C₆H₁₄N₂)[BX₃], where B = K⁺ and X = Br⁻ or I⁻, and B = Cs⁺ and X = Cl⁻, Br⁻ or a mixture of Cl⁻ and I⁻, display three-dimensional (3D) parent perovskite structures containing corner sharing metal halide octahedra that form a 3D framework. A correlation was found between the formation of a 1D or 3D perovskite structure and the sum of ionic radii of the metal and halide ions. The band gaps of the compounds fall in the range 3.42 eV to 5.33 eV. It was found that a 3D perovskite has a smaller band gap than a 1D perovskite, while the halide ion also affects the band gap.