RbFe ²⁺Fe ³⁺F ₆: Synthesis, structure, and characterization of a new charge-ordered magnetically frustrated pyrochlore-related mixed-metal fluoride

A new charge-ordered magnetically frustrated mixed-metal fluoride with a pyrochlore-related structure has been synthesized and characterized. The material, RbFe ₂F ₆ (RbFe ²⁺Fe ³⁺F ₆) was synthesized through mild hydrothermal conditions. The material exhibits a three-dimensional pyrochlore-related structure consisting of corner-shared Fe ²⁺F ₆ and Fe ³⁺F ₆ octahedra. In addition to single-crystal diffraction data, neutron powder diffraction and magnetometry measurements were carried out. Magnetic data clearly reveal strong antiferromagnetic interactions (a Curie-Weiss temperature of -270 K) but sufficient frustration to prevent ordering until 16 K. No structural phase transformation is detected from the variable-temperature neutron diffraction data. Infrared, UV-vis, thermogravimetric, and differential thermal analysis measurements were also performed. First-principles density functional theory (DFT) electronic structure calculations were also done. Crystal data: RbFe ₂F ₆, orthorhombic, space group Pnma (no. 62), a = 7.0177(6), b = 7.4499(6), c = 10.1765(8) Å, V = 532.04(8) Å ³, Z = 4.