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., BA2Cu0.5In0.5Cl4, BA2Ag0.5In0.5Cl4, BA2Ag0.5Sb0.5Cl4, and BA2Ag0.5Sb0.5Br4 [BA = butylammonium = CH3(CH2)3NH3+], each of which has the n = 1 Ruddlesden-Popper structure. In addition, the crystal structure of BA2Ag0.5Bi0.5Br4 is revisited and that of BA2PbCl4 is reported for the first time. Only BA2Ag0.5Sb0.5Cl4 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 BA2Ag0.5Bi0.5Br4 and BA2Cu0.5In0.5Cl4 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 BA2M0.5M'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.