Temperature-Dependent Mixed Valency in the Hexagonal Perovskite Cs₃NaFe₂Cl₉

The hexagonal perovskites Cs₃NaFe₂Cl₉ and Cs₃NaMnFeCl₉ have been synthesized and investigated. Both compounds adopt the 6H perovskite structure with P6₃/mmc symmetry. This structure consists of dimers of face-sharing octahedra arranged on the vertices of a triangular network. The transition metal ions occupy sites in these octahedra, leading to Fe₂Cl₉^4- and FeMnCl₉^4- bioctahedra, respectively. The bioctahedral clusters are sandwiched by layers of corner-sharing octahedra occupied by Na⁺ cations. Diffuse reflectance spectroscopy reveals optical transitions that arise from metal-to-metal charge transfer (Cs₃NaMnFeCl₉) and intervalence charge transfer (Cs₃NaFe₂Cl₉) excitations. In Cs₃NaFe₂Cl₉, magnetic susceptibility measurements reveal local ferromagnetic coupling (θ_CW = 16.7 K), mediated by the rapid exchange of an electron between the iron sites within each dimer. In contrast, the magnetic coupling between Fe³⁺ and Mn²⁺ in Cs₃NaMnFeCl₉ is antiferromagnetic (θ_CW = -41.4 K). At 100 K, the Mössbauer spectrum is dominated by a single type of iron that corresponds to Fe^2.5+, signaling electron exchange between iron sites that is faster than the Mössbauer time scale (∼100 ns). Upon further cooling, the Fe^2.5+ signal gives way to a 1:1 ratio of Fe²⁺ and Fe³⁺, as the thermally activated hopping slows down.