Ferroelectric structure and related properties of Pb sub 5 W sub 3 O sub 9 F sub (10).

Lead tungsten oxyfluoride, Pb sub 5 W sub 3 O sub 9 F sub (10), is a new ferroelectric nonlinear optic material. The structure is shown to contain independent WO sub 6, WO sub 3 F sub 3, and WO sub 2 F sub 4 octahedra and two independent PbO sub 4 F sub 5 and a PbO sub 4 F sub 4 polyhedron. The major local dipole contribution to the spontaneous polarization is associated with a tungsten atom that is displaced 0.172 (43)angstroms from the center of its octahedron. The corresponding predicted Curie temperature is 590(260)K, in agreement with the experimental value of 785(15)K. All six metal atoms undergo anharmonic thermal vibrations, which are fitted to fifth- and sixth-order in an expansion of the probability density function. The average interatomic distances are W-F = 1.798(8)angstroms, W-O = 1.944(18)angstroms, Pb-F = 2.68(19)angstroms and Pb-O = 2.72 (21)angstroms: standard deviations in parentheses are from Bessel's relation.