CRYSTAL STRUCTURE OF PYROELECTRIC PARAMAGNETIC BARIUM MANGANESE FLUORIDE, BAMNF4

BaMnF4, pyroelectric at room temperature and paramagnetic above 30°K, crystallizes in the orthorhombic system with space group A2 1am and lattice constants a = 5.9845±0.0003, b = 15.098±0.002, and c=4.2216± 0.0003 Å at 298°K. The integrated intensities of 5085 reflections within a reciprocal lattice hemisphere of radius (sinθ)/λ = 1.02 Å-1 were measured using PEXRAD. The crystal structure has been solved and refined by a combination of Patterson and Fourier series and the method of least squares, using 455 independent and significantly nonzero F (meas). The final agreement factor R, for atoms with anisotropic temperature coefficients, is 0.0466. The structure consists of MnF6 octahedra sharing corners to form puckered sheets parallel to (010). These sheets are linked by Ba2+ ions only, giving rise to excellent (010) cleavage. Within the sheets there are nearly linear -Mn-F-Mn-F- chains parallel to c. Parallel to a there is a second set of planar zigzag -Mn-F-Mn-F- chains, formed by adjacent shared fluorine atoms with Mn-F-Mn angles of 138.6°. The F-Mn-F angles in this chain are 98.3°. The remaining two fluorine atoms in each octahedron are adjacent. These atoms, on the outside of the sheets, are bonded to one Mn atom only. The octahedra are distorted with Mn-F distances ranging from 2.037 to 2.151 Å, with an average of 2.098 Å. Each barium ion is surrounded by a distorted trigonal prism of fluorine atoms, with two more located in the equatorial plane through rectangular prism faces, all Ba-F distances being less than 2.88 Å. A model is presented to account for the observed ferroelectricity in BaZnF 4 and the absence of ferroelectricity in isomorphous BaMnF 4, based on the separation of the Zn or Mn atoms in the puckered sheets. The experimental absolute polarization sense is in agreement with that calculated. The theoretical polarization for BaMnF4 of ll.5±1.5 μC cm-2 may be compared with the average value of 8.0 μC cm-2 for the ferro-electric members of the BaMF4 series.