title: Neutron Rietveld refinement of the incommensurate phase of the ordered perovskite Pb2CoWO6.
abstract: The incommensurate structure of lead cobalt tungstate has been refined by the Rietveld method on neutron data collected at 250 K. The space group is planar monoclinic 12/m(alpha0gamma)0s [a = 7.9602 (4), b = 5.6779 (3), c = 5.6967 (3) A, beta = 90.047 (5)degrees, q(inc) = 0.9000 (9)a* + 0.1735 (6)c*]. The use of powder diffraction techniques to investigate ferroelastic modulated phases is discussed and compared with a previous polydomain single-crystal structural analysis. The modulated displacements of light atoms have been determined, allowing an accurate description of the modulation of both the cations and the O-atom framework. The refinement suggests a displacive model for the phase transition, involving significant atomic shifts for Pb atoms and a quite complex mixing of tilt and deformation of the oxygen octahedra. The average character of this modulated structure is antiferroelectric.
authors.affiliation: Structures, propriétés et modélisation des solides, UMR CNRS, Ecole Centrale Paris, Châtenay, Malabry, France. baldinozzi@cps.ecp.fr
authors.forename: G
authors.initials: G
authors.lastname: Baldinozzi
authors.forename: G
authors.initials: G
authors.lastname: Calvarin
authors.forename: P
authors.initials: P
authors.lastname: Sciau
authors.forename: D
authors.initials: D
authors.lastname: Grebille
authors.forename: E
authors.initials: E
authors.lastname: Suard