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 Fe2Cl94- and FeMnCl94- 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 (θCW = 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 (θCW = -41.4 K). At 100 K, the Mössbauer spectrum is dominated by a single type of iron that corresponds to Fe2.5+, signaling electron exchange between iron sites that is faster than the Mössbauer time scale (∼100 ns). Upon further cooling, the Fe2.5+ signal gives way to a 1:1 ratio of Fe2+ and Fe3+, as the thermally activated hopping slows down.