Synthesis, crystal growth, and characterization of the orthorhombic BaTeW2O9: a new polymorph of BaTeW2O9†
A new polymorph of BaTeW2O9 with space group Pnma was identified during the exploration of the BaO-WO3–TeO2 system. Single-crystal X-ray diffraction measurements show that the material exhibits a 3D tunnel structure consisting of W–O6 octahedra connected with Te–O3 tetrahedra. Bulk single crystals of orthorhombic BaTeW2O9 with dimensions of 24 mm × 22 mm × 15 mm were grown by a top-seeded solution growth (TSSG) method using TeO2 as flux. Theoretical morphology of the crystal was obtained using the Bravais–Friedel and Donnay–Harker methods for comparison. The crystal shows a high transmittance from 360 nm to 5 μm as well as large birefringence (with Δn = 0.247 at 404.7 nm). The Raman spectrum shows that there is a Raman shift with high intensity and narrow peak width at 910.93 cm−1. TGA and DSC analysis indicate that the material melts incongruently at 783 °C. The thermal conductivity of the crystal was measured to be 1.23, 2.61 and 0.95 W m−1 K−1 at 50 °C along the a-, b- and c-axes, respectively.