Lead tungsten oxyfluoride, Pb5W3O9F10, is a new ferroelectric material that crystallizes in the tetragonal system with space groupI4 and four formulas in the unit cell. The lattice constants at 295 K area=14.583(3) andc=7.365(1) Aring; for lgr;(MoKagr;1,2)=0.709thinsp;30, 0.713thinsp;59 Aring;. The integrated intensities of all 10thinsp;757 reflections within a reciprocal hemisphere of radius (sinthinsp;thgr;)/lgr; le;0.91 Aring;minus;1were measured on a CADhyphen;4 diffractometer with MoKagr; radiation and resulted in 7080F2mge;2sgr;F2m. Correction for linear anisotropic radiation damage, followed by averaging the symmetry equivalent reflections, gave 2933 independent lang;thinsp;F2mrang;ge;2sgr;lang;thinsp;F2mrang; with internal agreement factorRintlang;Fmrang; =0.0289.The crystal structure was solved from the Patterson function and difference Fourier series and refined onwlang;thinsp;F2mrang; by the method of least squares. The final agreement factors between measurement and model wereR=0.0382 andwR=0.0329 on lang;thinsp;Fmrang;. The ashyphen;grown crystal contained equal volumes of plusmn;Zdomains. The three independent tungsten atoms form WO6, WO3F3, and WO2F4octahedra, and the three independent lead atoms form two different PbO4F5and a PbO4F4polyhedron. The major local dipole contribution to the spontaneous polarization is associated with a tungsten atom that is displaced 0.172(43) Aring; 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 either to fifth or sixth order in a Gramndash;Charlier expansion of the probability density function. The average interatomic distances are Wndash;F=1.798(8) Aring;, Wndash;O=1.944(18) Aring;, Pbndash;F=2.68(19) Aring;, and Pbndash;O=2.72(21) Aring;: standard deviations are from Besselrsquo;s relationship. The atomic rearrangement required in forming Pb5W3O9F10from PbF2and WO3is discussed, and similarities and contrasts between the related families for which Pb5W3O9F10, SrAlF5, and Sr3(FeF6)2provide the archetype structures are considered. All three families exhibit ferroelectric behavior.
展开▼
