CRYSTAL-GROWTH OF ALKALI-METAL TUNGSTEN BRONZES M(X)WO(3) (M=K, RB, CS), AND THEIR OPTICAL-PROPERTIES

Attempts have been made to grow crystals of alkali metal hexagonal (HT B) and tetragonal (TTB) tungsten bronzes (M(x)WO(3)) by chemical trans port in a temperature gradient. Different transport agents were used ( HgCl2, HgBr2, HgI2, Cl-2, PtCl2). Needle-shaped crystals of up to 6 mm in length could be grown using only HgCl2 and HgBr2. It was observed that the size of HTB crystals and the transport rate drop with increas ing content of alkali metal. TTB crystals larger than 0.1 mm in dimens ions could not be prepared. The electrical properties of different cry stals have been investigated by reflection spectroscopy in the spectra l range from 600 to 20 000 cm(-1). The spectral properties can be expl ained by using the Drude free carrier model. According to this there a re plasma frequencies which are shifted systematically towards larger values for larger x, and anisotropic effective masses m(E perpendicul ar to)approximate to 3m(E parallel to c) with m*(E perpendicular to c ) between 0.5 and 1.3 which also increase systematically with respect to x. An exclusion from Drude behaviour is observed for K-HTB for the E parallel to c polarization. For these cases the line profile can bet ter be described using a Lorentz-oscillator model.