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 Rh2X93– (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.