Synthesis and characterization of niobium-doped potassium tetragonal tungsten bronzes, KxNbyW1−yO3

Needle-shaped crystals of sizes up to 5 μm × 5 μm × 40 μm of nominal composition KxNbyW1−yO3 were synthesized by solid state method at 800 °C using appropriate amount of WO3, WO2, Nb2O5, and K2WO4. The samples were characterized with XRD, SEM, microprobe analysis, optical spectroscopy, and Raman spectroscopy. The XRD patterns of the samples show single phase of tetragonal tungsten bronze (TTB) type (P4/mbm, No. 127) up to y = 0.07. Structure refinements reveal an increase in cell parameter with increasing nominal niobium content within the TTB phase. The elemental compositions of the crystals determined by electron microprobe analysis also show an increase in Nb content with increasing y. With increasing Nb content the reflectivity minimum in the near infrared spectral range shifts towards lower wavenumber indicating the effect of decreasing carrier concentration. Pyrochlore type phase (KNbWO6) is obtained as a second phase when nominal composition y > 0.07.