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

Crystals of LixWO3 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 (PTBc) for x=0.45 and 0.4, mixed phase of PTBc and Perovskite tungsten bronze of tetragonal symmetry (PTBt) for x=0.35, 0.3 and 0.25 and of PTBt and Perovskite tungsten bronze of orthorhombic symmetry (PTBo) for x=0.1. The structure of PTBt 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. PTBc and PTBt) 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 PTBt and PTBc, 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” (wp) of about 12,900 and 13,700 cm−1, respectively.