Triply Ordered 6H Halide Perovskites: A Family of Triangular Lattice Antiferromagnets

Three new hexagonal perovskites with Cs₃M⁺M²⁺RhCl₉ (M⁺ = Na⁺, Ag⁺; M²⁺ = Mn²⁺, Fe²⁺) stoichiometry have been synthesized from solution precipitation reactions. These air-stable compounds crystallize as triply cation-ordered variants of the 6H perovskite structure. This structure contains octahedra that share a common face to form M²⁺RhCl₉^4- dimers that are arranged on a two-dimensional triangular network. M⁺ cations reside in octahedral holes located between dimer layers and connect M²⁺RhCl₉^4- dimers through corner-sharing linkages. The cation sites in the corner-sharing layers are fully occupied by Na⁺/Ag⁺, except for Cs₃AgMnRhCl₉, where a small amount of Ag⁺/Mn²⁺ antisite disorder is observed. The M²⁺RhCl₉^4- dimers adopt an ordered configuration that lowers the symmetry from P6₃/mmc to P6₃mc. Optical absorption in the near ultraviolet (UV) and visible regions is dominated by Rh d-to-d transitions and metal-to-metal charge transfer transitions. Variable temperature magnetic susceptibility measurements show no sign of long-range magnetic order down to 2 K. Curie-Weiss fits reveal relatively weak antiferromagnetic interactions between M²⁺ ions, with Weiss constants that range from -2.3 to -5.4 K. The strength of the antiferromagnetic interactions between M²⁺ ions increases as the covalency of the M²⁺-Cl bond increases. The ordering of three different cations in the 6H perovskite structure diminishes magnetic coupling between layers, leading to a new family of two-dimensional triangular lattice antiferromagnets.