Connectivity-Driven Electronic Structure and Charge Separation in Morpholinium-Based Bi ³⁺ /Sb ³⁺ Halides

High Resolution Image Download MS PowerPoint Slide Compositional mixing in organic–inorganic metal-halide double perovskite related materials leads to diverse structural motifs with distinct polyhedral connectivity. In A 2 MM′X 6 compounds, the organic A-site cation and alkali-metal M ion play a key role in determining the connectivity of M′X 6 (M′ = Bi 3+, Sb 3+ ) octahedra and electronic properties. Here, we report four morpholinium (MOR)-based halides: (MOR) 2 CsBiCl 6 ( 1 ), (MOR) 2 CsSbCl 6 ( 2 ), (MOR) 2 KBiCl 6 ( 3 ), and (MOR) 2 RbSbI 6 ( 4 ), characterized by single-crystal X-ray diffraction and DFT calculations. Despite identical composition, the compounds adopt distinct architectures: 1 is centrosymmetric with nearly regular octahedra, 2 crystallizes in a polar space group with distorted SbCl 6 units, 3 forms a 1D edge-sharing framework, whereas 4 adopts a 3D framework with different connectivity. Effective-mass calculations reveal anisotropic charge transport in 1 and 2, with higher electron than hole mobility and spatial separation of band-edge charge densities in 1, consistent with a direct Z-scheme heterojunction. Photoluminescence at 80 K reveals two emission bands (415/440 nm) with distinct recombination mechanisms arising from the band gap offset between inequivalent BiCl 6 units. Large Cl···Cl separations in 3 suppress orbital overlap and limit charge mobility. These results highlight the roles of cation size, lone-pair activity, and halide polarizability in directing structure and electronic behavior.