Bismuth-based halide double perovskite Cs2KBiCl6: Disorder and luminescence

A new bismuth-based halide double perovskite Cs2KBiCl6 was isolated successfully using solid-state reactions and investigated using X-ray and neutron diffraction. Rather than an ordered structure, the crystal structure consists of shifted Cs, K, Bi, and Cl sites from the ideal positions with fractional occupancy in compensation, leading to variable local coordination of Cs+ ions, as revealed by 133Cs solid-state nuclear magnetic resonance spectroscopy. Cs2KBiCl6 display volume hysteresis at 5-298 K range upon heating and cooling. The Cs2KBiCl6 has a direct bandgap of 3.35(2) eV and red-shift luminescence of around 600 nm upon Mn doping compared with the Na analogue. The stabilization of disordered structure in Cs2KBiCl6 is related to two factors including the large-sized K+ cation which prefers to coordinate with more than six Cl−, and the Bi3+ with 6s2 lone pair which has a preference for a local asymmetric environment. These findings could have general application and help to understand the structure and properties of halide perovskites.