Crystal structure of the antiferroelectric perovskite Pb₂MgWO₆

Abstract

Lead magnesium tungstate, Pb2MgWO6, Mr = 718.54. Phase I: cubic, Z = 4, Fm3m, a = 8.0058 (4)A, V = 513.1 (2) A 3, Dx = 9.30 Mg m -3 at 350 K, final Rwp = 4.5 and 7.7%, RBragg = 2.9 and 5.7% for neutron and Xray powder data, respectively. Phase II: orthorhombic, Pmcn (Pnma), Z = 4, a = 7.9440(4) and 7.9041 (3), b = 5.6866(3) and 5.7035(2), c = 11.4059(5) and 11.4442 (4) A, V= 515.3 (1) and 515.9 (1) A 3 at 294 and 80 K, respectively, Dx = 9.26 Mg m -3 at 294 K. Final Rwp = 4.0 and 8.5%, RBragg = 4.0 and 9.2% at 294 K and Rwp = 4.0 and 7.4%, RBragg = 2.9 and 8.4% at 80 K for neutron and X-ray powder data, respectively. To achieve the determination of the structures, X-ray and neutron powder diffraction data were refined together using the Rietveld profile method. The Pb main displacement in the orthorhombic phase from the ideal cubic positions is almost along the [0121o direction. The O displacements correspond to a weak distortion of the octahedra.