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 I2/­m(0)0s [a = 7.9602 (4), b = 5.6779 (3), c = 5.6967 (3) Å,  = 90.047 (5)°, = 0.9000 (9) + 0.1735 (6)]. 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.