Four Lead-free Layered Double Perovskites with the n = 1 Ruddlesden-Popper Structure

Herein we report the synthesis, structure, and band gaps of four layered halide double perovskites, i.e., BA₂Cu_0.5In_0.5Cl₄, BA₂Ag_0.5In_0.5Cl₄, BA₂Ag_0.5Sb_0.5Cl₄, and BA₂Ag_0.5Sb_0.5Br₄ [BA = butylammonium = CH₃(CH₂)₃NH₃⁺], each of which has the n = 1 Ruddlesden-Popper structure. In addition, the crystal structure of BA₂Ag_0.5Bi_0.5Br₄ is revisited and that of BA₂PbCl₄ is reported for the first time. Only BA₂Ag_0.5Sb_0.5Cl₄ has the tetragonal I4/mmm symmetry of the undistorted Ruddlesden-Popper structure. The other five compounds have orthorhombic structures due to tilts of the octahedra and orientational ordering of the butylammonium groups. As the lateral dimensions of the inorganic layer decrease, the c/a ratio increases due to decreased interdigitation of the alkyl ends of the butylammonium cations. This structural feature may help to explain the increased stability of the bromide phases with respect to the chloride phases. There are features in the diffraction patterns of BA₂Ag_0.5Bi_0.5Br₄ and BA₂Cu_0.5In_0.5Cl₄ that suggest ordering of octahedral cations within the layers, but in those compounds there appears to be a high concentration of stacking faults between layers that limits long-range, three-dimensional ordering of cations. In the other cases the scattering powers of the cations (Ag/Sb and Ag/In) are too similar to say anything definitive about cation ordering. The band gaps of these compounds range from 2.65 to 4.27 eV, with the bromide compositions possessing smaller band gaps than the chlorides. The band gaps of layered BA₂M_0.5M'_0.5X₄ compositions studied here are roughly 0.5-0.8 eV larger than analogous Cs₂MM'X₆ cubic double perovskites due to a combination of dimensional reduction (3D → 2D), distortions of the octahedral environment around the M/M' ions, and octahedral tilting distortions.