Asymmetric cationic coordination environments in new oxide materials:: Synthesis and characterization of Pb4Te6M10O41 (M = Nb5+ or Ta5+)

Two new isostructural tellurites, Pb4Te6M10O41 (M = Nb5+ or Ta5+), have been synthesized by standard solid-state techniques using PbO, Nb2O5 (or Ta2O5), and TeO2 as reagents. The structures of Pb4Te6Nb10O41 and Pb4Te6Ta10O41 were determined by single-crystal and powder X-ray diffraction. The materials exhibit a three-dimensional framework consisting of layers of corner-shared NbO6 Octahedra connected by TeO3 and PbO6 polyhedra. The Nb5+, Te4+, and Pb2+ cations are in asymmetric coordination environments attributable to second-order Jahn-Teller effects. The Nb5+ cations undergo an intraoctahedral distortion either toward a face or a corner, whereas the Te4+ and Pb2+ cations are in distorted environments attributable to their lone pair. In addition, the TeO3 polyhedra strongly influence the direction of the Nb5+ intraoctahedral distortion. Infrared and Raman spectroscopy, thermogravimetric analysis, and dielectric measurements are also presented. Crystal data: Pb4Te6Nb10O41, monoclinic, space group C2/m (No. 12), with a = 23.412(3) Angstrom, b = 20.114(3) Angstrom, c = 7.5008(10) Angstrom, beta = 99.630(4)degrees, V = 3482.4(8) Angstrom(3), and Z = 4; Pb4Te6Ta10O41, monoclinic, space group C2/m (No. 12), with a = 23.340(8) Angstrom, b = 20.068(5) Angstrom, c = 7.472(2) Angstrom, beta = 99.27(3)degrees, V = 3453.8(2) Angstrom(3), and Z = 4.