Multi-B-Site Cation Engineering in Stable Thiocyanate-Templated Layered Double-Perovskite Halides

Hybrid organic-inorganic perovskite halides offer a versatile structural platform owing to their flexible bonding nature. Beyond organic-cation engineering, halide-site engineering using molecular anions has recently emerged as an effective approach to alter the perovskite bonding network; however, accessible compositional and structural diversity remains limited. Here, we report, for the first time, hybrid layered Ag-Bi double perovskites, MA4AgBiX4(SCN)4 (X = Br, I), in which incorporation of thiocyanate (SCN-) at the halide-site reduces perovskite dimensionality. While the previously reported single-metal-cation compound, MA2PbI2(SCN)2, offers a unique van der Waals-coupled structural platform, its rapid decomposition, limited compositional space, and lack of bromide analogue hindered further development. First-principles calculations suggest that the structural instability of the lead-based bromide analogue arises from a bromide-induced lifting of the axial SCN ligand and resultant increased interlayer hydrogen-bond distance. Transitioning to an Ag-Bi architecture mitigates this destabilizing effect, as Ag+ (4d105s0) possesses isotropic s-orbital character and enhanced angular flexibility compared to Pb2+ (6s26p0) with a stronger orientational preference. This strategy yields a full range of solid solutions with band gaps tunable from 1.95 to 2.30 eV. The resulting materials, particularly the bromide-rich phases, exhibit markedly increased environmental robustness, demonstrating that multi-B-site cation design can expand the structural diversity of stable pseudohalide-based hybrid perovskites.