Multi-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. In particular, a remarkable number of novel layered perovskites have been realized by incorporating bulky organic cations. Not only organic cation engineering but also halide engineering using molecular anions has recently emerged as an effective approach to drastically alter the perovskite bonding network, yet the accessible compositional and structural diversity remains very limited. Here, we report, for the first time, hybrid layered Ag-Bi double perovskites, MA 4 AgBiX 4 (SCN) 4 (X = Br, I), in which incorporation of thiocyanate (SCN⁻) at the halide site reduces the dimensionality of the perovskite framework. While the previously reported single metal cation compound, MA 2 PbI 2 (SCN) 2 , offers a unique van der Waals-coupled structural platform, its rapid decomposition under ambient conditions and limited compositional space have hindered further development. First-principles calculations reveal 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. By transitioning to an Ag-Bi architecture, we demonstrate that the incorporating Ag + (4d 10 5d 0 ) with isotropic s-orbital character and thus enhanced angular flexibility, compared to Pb 2+ (6s 2 6p 0 ) having orientational preference, effectively mitigate this destabilizing effect. This strategy enables the synthesis of 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 a marked increase in environmental robustness. These results establish multi-cation design as a versatile and stable platform for expanding the structural space of pseudohalide-based hybrid perovskites.