Exploring the Electronic Dimensionality of Ternary and Quaternary Rhodium Halides

The synthesis, crystal structures, and optical properties of four ternary and six quaternary halides containing the Rh³⁺ ion are reported here. Rb₃RhCl₆ adopts a monoclinic structure with isolated [RhCl₆]^3– octahedra. Rb₃Rh₂Cl₉, Cs₃Rh₂Cl₉, and Cs₃Rh₂Br₉ crystallize in a vacancy-ordered variant of the 6H hexagonal perovskite structure, which contains isolated Rh₂X₉^3– (X = Cl and Br) dimers of face-sharing octahedra. Cs₂AgRhCl₆ and Cs₂NaRhCl₆ adopt the 12R rhombohedral perovskite structure, featuring [M₂RhCl₁₂]^7– face-sharing octahedral trimers connected to one another through rhodium-centered octahedra. A₄AgRhCl₈ and A₄AgRhBr₈ (A = CH₃CH₂CH₂CH₂NH₃⁺ and (CH₃)₂CHCH₂CH₂NH₃)⁺) crystallize in a cation-ordered variant of the n = 1 Ruddlesden–Popper structure, which features layers of corner-connected octahedra with a chessboard ordering of Ag⁺ and Rh³⁺ ions separated by double layers of organic cations. The diffuse reflectance spectra of all compositions studied feature peaks in the visible region that can be attributed to spin-allowed d-to-d transitions and peaks in the UV region that arise from charge transfer transitions. Electronic structure calculations reveal moderate Rh–X–Ag hybridization when rhodium- and silver-centered octahedra share corners but minimal hybridization when they share faces. Many of the compositions studied have an electronic structure that is effectively zero-dimensional, but Cs₂AgRhCl₆ is found to possess a two-dimensional electronic structure. The results are instructive for controlling the electronic dimensionality of compositionally complex halide perovskite derivatives.