Elucidating the role of lone-pair ns² electrons on B-site cation disordering in double halide perovskites Cs₂AgBiCl₆ and Cs₂AgInCl₆

The intrinsic B-site antisite defects fundamentally constrain the performance of double halide perovskites Cs₂AgBiCl₆ and Cs₂AgInCl₆. The resulting partially disordered or fully disordered structures exhibit distinct optoelectronic properties. Herein, we performed first-principles calculations to explore the accessibility of B-site cation disordering in Cs₂AgBiCl₆ and Cs₂AgInCl₆ and its effects on the electronic structures of both these double perovskites. It was revealed that the formation of cation-disordered Cs₂AgBiCl₆ required a lower energy cost than cation-disordered Cs₂AgInCl₆. Generally, the lone-pair electrons of Bi³⁺ make its electron configuration distinct from Ag⁺, which results in much higher energy cost for electronic relaxation during cation disordering of Cs₂AgBiCl₆. However, the more significant lattice relaxation of Cs₂AgBiCl₆ effectively reduced the energy cost of cation disordering. Furthermore, the resulting remarkable ionic relaxation and change in electronic redistribution in the cation-disordered Cs₂AgBiCl₆ effectively protected its electronic structure from the influence of cation disordering. Alternatively, cation disordering significantly influenced the electronic structure of Cs₂AgInCl₆. Our results provide an in-depth understanding on the lone-pair effect on cation disordering and electronic properties of double halide perovskites, which may be useful in the development and design of high-performance optoelectronic and photovoltaic devices based on double perovskites.