CRYSTAL-STRUCTURE OF THE ANTIFERROELECTRIC PEROVSKITE PB2MGWO6

Lead magnesium tungstate, Pb2MgWO6, M(r) = 718.54. Phase I: cubic, Z = 4, Fm3m, a = 8.0058(4)Angstrom, V = 513.1(2)Angstrom(3), D-x = 9.30 M g m(-3) at 350 K, final R(wp) = 4.5 and 7.7%, R(Bragg) = 2.9 and 5.7% for neutron and X-ray powder data, respectively. Phase II: orthorhombi c, 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)Angstrom, V = 515.3(1) and 515. 9(1)Angstrom(3) at 294 and 80 K, respectively, D-x = 9.26 Mg m(-3) at 294 K. Final R(wp) = 4.0 and 8.5%, R(Bragg) = 4.0 and 9.2% at 294 K an d R(wp) = 4.0 and 7.4%, R(Bragg) = 2.9 and 8.4% at 80 K for neutron an d X-ray powder data, respectively. To achieve the determination of the structures, X-ray and neutron powder diffraction data were refined to gether using the Rietveld profile method. The Pb main displacement in the orthorhombic phase from the ideal cubic positions is almost along the [012](o) direction. The O displacements correspond to a weak disto rtion of the octahedra.