Exploring the Electronic Dimensionality of Ternary and Quaternary Rhodium Halides
David Liu, Noah P. Holzapfel, Alexander Milder, Patrick M. Woodward
Abstract
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.