Synthesis and Characterization of the Aurivillius Phase CoBi₂O₂F₄

The new CoBi₂O₂F₄ compound was synthesized by a hydrothermal method at 230 °C. Single-crystal X-ray diffraction data were used to determine the crystal structure. The compound is layered and belongs to the Aurivillius family of compounds. The present compound is the first oxo-fluoride Aurivillius phase containing Co²⁺. Inclusion of a d-block cation with such a low oxidation state as 2+ was achieved by partially replacing O^2- with F^- ions. The crystal structure is best described in the tetragonal noncentrosymmetric space group I4̅ with unit-cell parameters a = 3.843(2) Å and c = 16.341(8) Å. The crystal structure consists of two main building units: [BiO₄F₄] distorted cubes and [CoF₆] octahedra. Interestingly, since the octahedra [CoF₆] tilt between four equivalent positions, the F atoms occupy a 4-fold split position at room temperature. For the investigation of the structural disorder, Raman scattering data were collected in the range from 10 K to room temperature. As the temperature decreases, sharper phonon peaks appear and several modes clearly appear, which indicates a reduction of the disorder. Magnetic susceptibility and heat capacity measurements evidence long-range antiferromagnetic ordering below the Néel temperature of ∼50 K. The magnetic susceptibility is in agreement with the Curie-Weiss law above 75 K with a Curie-Weiss temperature of θ_CW = -142(2) K.