Theoretical Exploration of the Physical-Chemical Properties of Divalent (<i>np</i> <sup>2</sup>) Cation Mixing in Double Cs<sub>2</sub>AgBiBr<sub>6</sub> Perovskite.

Abstract

Lead-free halide double perovskites have emerged as promising alternatives to conventional lead-based materials for photovoltaic applications, as they combine environmental compatibility with structural stability. However, their indirect band gaps limit optoelectronic performance, motivating compositional and structural optimization to achieve higher efficiency. In this work, we used density functional theory calculations to investigate complex mixed-halide double perovskites with general composition Cs₂Ag _<i>x</i> Bi _<i>x</i> <i>B</i> _<i>y</i> <i>B</i>' _<i>z</i> Br₆, where <i>B</i>,<i>B'</i> = Ge, Sn, or Pb. By coupling electronic-structure calculations with high-throughput stress-tensor optimizations across thousands of configurations, we identified the energetic and structural principles governing their stability and electronic properties. The results revealed a narrow energy distribution, indicating high structural flexibility and entropy-driven stabilization. Substitutional trends are dictated by ionic size, with larger cations reducing octahedral distortions and promoting lattice symmetry. Although all ternary mixtures exhibited positive excess energies, Sn- and Pb-rich compositions were energetically favored. The decomposition of the bond energy consistently linked the strength of the metal-halide interaction to the stability of the lattice, with increasing Ag content weakening the overall bonding. In particular, partial substitution of Ge, Sn, or Pb at the <i>B</i>-site of pristine Cs₂AgBiBr₆ enhanced electronic transitions and induced nonlinear bowing effects, demonstrating heterovalent substitution as an effective strategy for tuning stability and optoelectronic performance in lead-free perovskite absorbers.