Synthesis, Crystal Structure, and Optical Gap of Two-Dimensional Halide Solid Solutions CsPb 2 (Cl 1- x Br x ) 5 .

Exploring new perovskite-related solid-state materials and the investigating composition-dependent structural and physical properties are highly important for advanced functional material development. Herein, we present the successful hydrothermal synthesis of tetragonal CsPb2 Cl5 and the anion-exchange phase formation of CsPb2 (Cl1- x Br x )5 ( x = 0-1) solid solutions. The CsPb2 (Cl1- x Br x )5 crystal structures, which crystallize in the tetragonal system, space group I4/ mcm, with parameters similar to those of CsPb2 Cl5 , have been determined by Rietveld analysis. The optical band gap was obtained by UV-vis spectroscopy, and the band structure was further calculated by the full-potential method within the generalized gradient approximation. It was revealed that the band gap in CsPb2 (Cl1- x Br x )5 solid solutions can be tuned over the range of 4.5-3.8 eV by anion substitution.