Exploring the Halide Exchange Engineering on Lead-free Cs2AgSbCl6−δBrδ Mixed-Halide Double Perovskite: A DFT Study

Halide perovskites, commonly studied in optoelectronics, have an ABX3 structure, where A+ is a monovalent cation, B2+ is a divalent cation, and X– is a halide ion. The use of the Pb2+ ion in the B site has generated some concern regarding its toxicity in a possible large-scale application. To address this, replacing Pb2+ with heterovalent elements is explored to obtain double halide perovskites, denoted as A2B′B″X6, where B′ and B″ are trivalent and monovalent cations, respectively. Anion exchange reactions can tailor the optical and electronic properties of these structures by band gap energy control. This study seeks to reveal the strong correlation between the structure composition of Cs2AgSbCl6−δBrδ mixed-halide double perovskites by means of computational simulations. These structures consist of three-dimensional (3D) lattices of [SbCl6−δBrδ] and [AgCl6−δBrδ] distorted octahedral clusters. For different δ values, the compounds exhibit different space groups: I4mm (C4v) for δ = 2 and 4, remaining I4mm (C4v) for δ = 1 and 5, and showing R3m (C3v) symmetry for δ = 3. Incorporating Br– significantly reduces structural organization in short and long ranges. All Cs2AgSbCl6−δBrδ compounds exhibit indirect band gap energy at the X → L point, reducing from 2.72 to 1.77 eV with the increased Br content.