Intrinsic Instability of Neutral-Molecule-Based Halide Perovskites: A Case Study on CH4BiCl3 and Beyond

Abstract The pursuit of Pb-free halide perovskites has driven innovative strategies, including incorporating neutral molecules (such as CH4 and CH3NH2) as A-site occupants while fully substituting Pb with Bi or Sb to maintain stoichiometric perovskite structure with 3D electronic dimensionality. However, theoretical claims of stability for neutral-molecule-based halide perovskites remain unverified and potentially flawed. In this work, we critically reassess such compounds using CH4BiCl3 as a central case study and extend the analysis to other representative systems (NH3BiCl3, CH3NH2BiX3, X = Cl, Br, I) through rigorous density functional theory calculations. Our results reveal methodological artifacts in prior studies, including improper k-point sampling that masks antibonding states and misinterpretation of decomposition energy. The calculated strongly negative decomposition enthalpies and observed structural collapse across these compounds confirm their thermodynamic and dynamical instability. Our results strongly indicate that Pb-free halide perovskites are unlikely to be stabilized by incorporating neutral molecules due to weak van der Waals interactions, a finding that extends beyond the CH4BiCl3 case, thereby highlighting the importance of ionic A-site species for viable Pb-free designs.