Mixed-metal tellurites:: Synthesis, structure, and characterization of Na1.4Nb3Te4.9O18 and NaNb3Te4O16

Two new mixed-metal tellurites, Na1.4Nb3Te4.9O18 and NaNb3Te4O16, have been synthesized by standard solidstate techniques using Na2CO3, Nb2O5, and TeO2 as reagents. The structures of Na1.4Nb3Te4.O-9(18) and Na1.4Nb3Te4O18 were determined by single-crystal X-ray diffraction. Both of the materials exhibit three-dimensional structures composed of NbO6 octahedra, TeO4, and TeO3 Polyhedra. The Nb5+ and Te4+ cations are in asymmetric coordination environments attributable to second-order Jahn-Teller (SOJT) effects. The Nb5+ cations undergo an intraoctahedral distortion toward a corner (local C-4 direction), whereas the Te4+ cations are in distorted environments owing to their nonbonded electron pair. Infrared and Raman spectroscopy, UV-vis diffuse reflectance spectroscopy, thermogravimetric analysis, and dielectric measurements were also performed on the reported materials. Crystal data: Na1.4Nb3Te4.O-9(18), monoclinic, space group C2/m (No. 12), with a = 32.377(5) angstrom, b = 7.4541 (11) A, c = 6.5649(g) angstrom, beta = 95.636(5)degrees, V = 1576.7(4) angstrom(3) and Z = 4; NaNb3Te4O16, monoclinic, space group P211m (No. 11), with a = 6.6126(13) angstrom, b = 7.4738(15) angstrom, c = 14.034(3) angstrom, = 102.98(3)angstrom, beta = 675.9(3)degrees angstrom(3), and Z= 2.