Acta Crystallographica Section B

Structural Science

Volume 56, Part 4 (August 2000)

research papers

Acta Cryst. (2000). B56, 570-576 [ doi:10.1107/S0108768100000677 ]

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

G. Baldinozzi, G. Calvarin, Ph. Sciau, D. Grebille and E. Suard

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 I2/m( $\alpha$ 0 $\gamma$ )0s [a = 7.9602 (4), b = 5.6779 (3), c = 5.6967 (3) Å, $\beta$ = 90.047 (5)°, $\bf q_{\rm inc}$ = 0.9000 (9) $\bf a^*$ + 0.1735 (6) $\bf c^*$ ]. 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.

Keywords: Neutron diffraction; Incommensurate structures; Perovskite.