Synthesis and Characterization of the Aurivillius Phase CoBi<sub>2</sub>O<sub>2</sub>F<sub>4</sub>

The new CoBi<sub>2</sub>O<sub>2</sub>F<sub>4</sub> 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 Co<sup>2+</sup>. Inclusion of a d-block cation with such a low oxidation state as 2+ was achieved by partially replacing O<sup>2–</sup> with F<sup>–</sup> ions. The crystal structure is best described in the tetragonal noncentrosymmetric space group <i>I</i>4̅ with unit-cell parameters <i>a</i> = 3.843(2) Å and <i>c</i> = 16.341(8) Å. The crystal structure consists of two main building units: [BiO<sub>4</sub>F<sub>4</sub>] distorted cubes and [CoF<sub>6</sub>] octahedra. Interestingly, since the octahedra [CoF<sub>6</sub>] 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<sub>CW</sub> = −142(2) K.