Thermodynamic and Electronic Effects of B-Site Alloying in Halide Perovskites

Inorganic halide perovskites (IHPs) are promising optoelectronic materials, but the thermodynamic and electronic effects of B-site alloying across a broad chemical space remain poorly understood. Here, we perform a high-throughput study combining density functional theory (DFT) and machine-learned potentials (MLIPs) across more than 6700 single (ABX 3 ) and alloyed (AA′(B x B′ 1– x ) 2 X 6 ) IHP compositions to evaluate stabilities and band gaps. Our stability analysis shows that B-site alloying generally destabilizes perovskites relative to their constituent ABX 3 s, with 65.1% of alloys exhibiting positive mixing enthalpies. Despite this trend, alloy stability is highly dependent on its most stable constituent ABX 3, revealing a practical design rule: stable ABX 3 s can improve the stability of unstable ABX 3 s through alloying. Analysis of electronic properties reveals two distinct band gap behaviors from B-site alloying. Alloys composed of alkaline-earth metals (AEMs) and post-transition metals (PTMs) exhibit continuous band gap evolution driven by band edge energy alignment and spatial delocalization of s- and p-orbitals, enabling strong band edge coupling. In contrast, alloys involving transition metals (TMs) frequently exhibit discontinuous band gap changes due to the energetic misalignment and spatial localization of TM d-orbitals, which weakly couple to the s- and p-orbitals of AEMs and PTMs and d-orbitals of other TMs. Together, our findings provide design principles for improving the thermodynamic stability of halide perovskite alloys and predicting whether the band gap evolution will be continuous or discontinuous.