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 Rh 3+ ion are reported here. Rb 3 RhCl 6 adopts a monoclinic structure with isolated [RhCl 6 ] 3– octahedra. Rb 3 Rh 2 Cl 9, Cs 3 Rh 2 Cl 9, and Cs 3 Rh 2 Br 9 crystallize in a vacancy-ordered variant of the 6H hexagonal perovskite structure, which contains isolated Rh 2 X 9 3– (X = Cl and Br) dimers of face-sharing octahedra. Cs 2 AgRhCl 6 and Cs 2 NaRhCl 6 adopt the 12R rhombohedral perovskite structure, featuring [M 2 RhCl 12 ] 7– face-sharing octahedral trimers connected to one another through rhodium-centered octahedra. A 4 AgRhCl 8 and A 4 AgRhBr 8 (A = CH 3 CH 2 CH 2 CH 2 NH 3 + and (CH 3 ) 2 CHCH 2 CH 2 NH 3 ) + ) 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 Rh 3+ 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 Cs 2 AgRhCl 6 is found to possess a two-dimensional electronic structure. The results are instructive for controlling the electronic dimensionality of compositionally complex halide perovskite derivatives.