Chemical Bonding Rules for Stacking-Sequence Stability in Bismuth Oxychloride Intergrowths and Their Consequences for Band Structure

Metal oxyhalides are versatile materials, exhibiting properties relevant to photocatalysis, solid-state battery systems, ferromagnetism, multiferroics, and nonlinear optics. Here, mixed-metal oxyhalide intergrowths, layered materials formed by intercalating SrBi3O4Cl3 and Bi2GdO4Cl blocks along the c direction within a crystal, are reported. They are synthesized through a flux-based approach in which the SrBi3O4Cl3-to-Bi2GdO4Cl ratio can be precisely controlled. Despite the structural and compositional flexibility of such intergrowths, there is no general framework for predicting which local structural arrangements are preferred. Density functional theory combined with Bader charge analysis shows that preferred stacking sequences are those that minimize charge disproportionation across distinct chlorine environments, consistent with Pauling's fifth rule, and we support this bonding-based framework through computational cation substitution that shifts the ionic-covalent balance of the metal-chloride bonding. With the preferred stacking arrangement established, this same system further reveals that intergrowth layering and Sr/Bi site occupancy modulate the interaction between the Bi3+ lone pair and anion p orbitals. Density functional theory calculations reveal that intergrowth formation destabilizes the O-2p orbitals, raising the valence band maximum for visible-light absorption, with the lone-pair interaction and layering-driven internal electrostatic field acting as competing effects on the resulting band gap. Significantly, this experimental-computational study establishes a transferable, bonding-based framework for stacking-sequence stability in layered oxyhalide intergrowths, while also clarifying how the same structural levers used to control stability can be tuned to engineer band structure.