Neutron Rietveld refinement of the incommensurate phase of the ordered perovskite Pb2CoWO6.

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.