Unique Behavior of Halide Double Perovskites with Mixed Halogens

Engineering halide double perovskite (A₂M⁺M³⁺X^VII₆) by mixing elements is a viable way to tune its electronic and optical properties. In spite of many emerging experiments on halide double perovskite alloys, the basic electronic properties of the alloys have not been fully understood. In this work, we chose Cs₂AgBiCl₆ as an example and systematically studied electronic properties of its different site alloys Cs₂NaAg_1-BiCl₆, Cs₂AgSbBi_1-Cl₆, and Cs₂AgBi(BrCl_1-)₆ (x = 0.25, 0.5, 0.75) by first-principles calculations. Interestingly, the halogen site alloy shows opposite behavior to M⁺ and M³⁺ cation site alloys; that is, Cs₂AgBi(BrCl_1-)₆ displays virtual crystal behavior without substantial broadening, while Cs₂NaAg_1-BiCl₆ and Cs₂AgSbBi_1-Cl₆ show split-band behaviors with substantial broadening, which indicates that lifetimes of electrons and holes in Cs₂AgBi(BrCl_1-)₆ would be longer than those in Cs₂NaAg_1-BiCl₆ and Cs₂AgSbBi_1-Cl₆. We further found that long lifetimes of electrons and holes are common for mixed halide perovskites. Moreover, the band alignment is provided to determine the band gap change of alloys and to understand the transport of electrons and holes when these pure compounds form heterostructures. Our systematical studies should be helpful for future optoelectronic applications of halide perovskites.