Mixed-Metal Tellurites:  Synthesis, Structure, and Characterization of Na_1.4Nb₃Te_4.9O₁₈ and NaNb₃Te₄O₁₆

Two new mixed-metal tellurites, Na_1.4Nb₃Te_4.9O₁₈ and NaNb₃Te₄O₁₆, have been synthesized by standard solid-state techniques using Na₂CO₃, Nb₂O₅, and TeO₂ as reagents. The structures of Na_1.4Nb₃Te_4.9O₁₈ and NaNb₃Te₄O₁₆ 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 C₄ 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:  Na_1.4Nb₃Te_4.9O₁₈, monoclinic, space group C2/m (No. 12), with a = 32.377(5) Å, b = 7.4541(11) Å, c = 6.5649(9) Å, β = 95.636(5)°, V = 1576.7(4) ų, and Z = 4; NaNb₃Te₄O₁₆, monoclinic, space group P2₁/m (No. 11), with a = 6.6126(13) Å, b = 7.4738(15) Å, c = 14.034(3) Å, β = 102.98(3)°, V = 675.9(3) ų, and Z = 2.