Connectivity-Driven Electronic Structure and Charge Separation in Morpholinium-Based Bi³⁺/Sb³⁺ Halides

Compositional mixing in organic-inorganic metal-halide double perovskite related materials leads to diverse structural motifs with distinct polyhedral connectivity. In A₂MM'X₆ compounds, the organic A-site cation and alkali-metal M ion play a key role in determining the connectivity of M'X₆ (M' = Bi³⁺, Sb³⁺) octahedra and electronic properties. Here, we report four morpholinium (MOR)-based halides: (MOR)₂CsBiCl₆ (1), (MOR)₂CsSbCl₆ (2), (MOR)₂KBiCl₆ (3), and (MOR)₂RbSbI₆ (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 SbCl₆ 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 BiCl₆ 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.