Cs2AgBiBr6−xClx solid solutions – band gap engineering with halide double perovskites

Abstract

The halide double perovskites Cs₂AgBiBr₆ and Cs₂AgBiCl₆ form a complete solid solution. The cubic Fmm space group symmetry and near complete rock salt ordering of Ag⁺ and Bi³⁺ are retained for all compositions, and the lattice parameter varies linearly between the two end-members. The band gap increases linearly between Cs₂AgBiBr₆ (E_g = 2.19 eV) and Cs₂AgBiBrCl₅ (E_g = 2.61 eV), but deviates upward from a Vegard's Law behavior once the chloride content exceeds ∼85%. The Cs₂AgBiCl₆ end-member has a band gap, E_g = 2.77 eV, that is 0.10 eV larger than the extrapolated value from a Vegard's Law fit. Analysis of the powder X-ray diffraction peak shapes shows that microstrain increases steadily as the Cl⁻ content increases, before dropping precipitously for the Cs₂AgBiCl₆ end-member. The combined results give no evidence for long range ordering of halide ions.