Perovskite tungsten bronze-type crystals of LixWO3 grown by chemical vapour transport and their characterisation

Crystals of Li_xWO₃ with nominal compositions, x=0.1, 0.25, 0.3, 0.35, 0.4 and 0.45 were grown by chemical vapour transport method using HgCl₂ as transporting agent. A complete transport was achieved with a temperature gradient T₁/T₂=800/700 °C revealing bluish-black crystals of sizes up to a few 10th of a millimeter. X-ray powder diffraction and infrared (IR) absorption spectra show Perovskite tungsten bronze of cubic symmetry (PTB_c) for x=0.45 and 0.4, mixed phase of PTB_c and Perovskite tungsten bronze of tetragonal symmetry (PTB_t) for x=0.35, 0.3 and 0.25 and of PTB_t and Perovskite tungsten bronze of orthorhombic symmetry (PTB_o) for x=0.1. The structure of PTB_t is explained by the off centring of the W-ions along c and tilting of the WO₆ octahedra around c. Crystal slices of mixed phase (i.e. PTB_c and PTB_t) reveal bright and dark areas on a sub-millimeter scale which are separated by sharp interfaces. Laser ablation inductively coupled plasma optical emission (LA ICP OES) analysis on small spot sizes show the separation into Li contents of x=0.18 (bright areas) and x=0.38 (dark areas) as threshold compositions of PTB_t and PTB_c, respectively. Polarized reflectivity using a microscope technique in the bright area of the crystals indicates strong anisotropic absorption effects with maximum between 1000 and 6000 cm⁻¹, which are related to optical excitations of polarons. Crystals of composition x=0.4 and 0.45 appear optically homogeneous and show an effective “free carrier-type plasma frequency” (w_p) of about 12,900 and 13,700 cm⁻¹, respectively.