Ferroelectric Paraelastic Paramagnetic Barium Cobalt Fluoride, BaCoF4, Crystal Structure

Abstract

Barium cobalt fluoride, BaCoF₄, ferroelectric and paramagnetic at room temperature, crystallizes in the orthorhombic system with lattice constants $\text{a\hspace{0.17em}=\hspace{0.17em}5.8519\hspace{0.17em}±\hspace{0.17em}0.0003, b\hspace{0.17em}=\hspace{0.17em}14.628\hspace{0.17em}±\hspace{0.17em}0.002,}\mathrm{and}\text{c\hspace{0.17em}=\hspace{0.17em}4.2102\hspace{0.17em}±\hspace{0.17em}0.0003 Å}$ at 298°K and space group ${\text{A2}}_1\mathrm{am}$ . The integrated intensities of 1526 reflections within a hemisphere of reciprocal space of radius $(\sin {\text{θ)\hspace{0.17em}/\hspace{0.17em}λ\hspace{0.17em}=\hspace{0.17em}1.02 Å}}^{\text{−1}}$ were measured with one crystal, and 2986 reflections with a second crystal of BaCoF₄, using PEXRAD. The crystal structure was refined by the method of least squares, using 386 symmetry‐independent Fmeas from the first crystal and 875 from the second. The combined set of 1261 Fmeas, together with anisotropic thermal coefficients for all atoms, resulted in a final agreement factor $\text{R\hspace{0.17em}=\hspace{0.17em}0.0679}$ . The structure is isomorphous with BaMnF₄ and contains CoF₆ octahedra sharing corners to form puckered sheets parallel to (010). These sheets are linked by Ba²⁺ ions only. The octahedra are distorted, with Co–F distances ranging from 1.972 to 2.109 Å, with an average of 2.040 Å. The crystal structure is consistent with the previously presented model for ferroelectric reversal in the BaMF₄ compounds. Both the sense and magnitude (8.0 ± 0.3 μC cm⁻²) of the observed absolute polarization is in good agreement with that calculated from a point‐charge model (8.6 ± 0.6 μC cm⁻²).