ACrF₃ Jahn-Teller-Distorted Fluoroperovskites: Expanding to RbCrF₃ and CsCrF₃

The Jahn-Teller effect significantly affects the CrF₆ octahedra in Cr(II) fluoroperovskites. Here, we report the synthesis, crystal structures, and magnetic properties of RbCrF₃ and CsCrF₃, thereby completing a comprehensive investigation of the ACrF₃ fluoroperovskites. Powder samples are prepared using a wet-chemical method, which allows stabilization of Cr(II). RbCrF₃ predominantly adopts the P4/mbm space group; however, the absence of the expected (211) peak suggests a limited correlation length, and we demonstrate that this is a result of stacking faults in the rotation of the distorted CrF₆ octahedra along the c-axis. We estimate around 5-10% of I4/mcm-type stacking in P4/mbm for RbCrF₃. In contrast, CsCrF₃ showcases a two-phase coexistence of I4/mcm and P4/mbm polymorphs and, as such, parallels to KCuF₃. We discuss and highlight the interplay between the impact of A-site cations and the Jahn-Teller effect acting on CrF₆ octahedra. Antiferromagnetic interactions dominate the paramagnetic regime, and RbCrF₃ and CsCrF₃ both have a Weiss temperature of -12 K, associated with Néel temperatures of 50 and 47 K, respectively. The effective paramagnetic moments of 4.65 and 4.68 μ_B for RbCrF₃ and CsCrF₃ are in good agreement with S = 2 spin configuration.