Solid-state synthesis in the system Na0.8NbyW1-yO3 with 0≤y≤0.4: A new phase, Na0.5NbO2.75, with perovskite-type structure

Series of compounds in the system NaxNbyW1-yO3 were prepared according to the appropriate molar ratio of Na2WO4, WO3, WO2 and Nb2O5 with x=0.80 and 0.0≤y≤0.4 at 600 deg. C in evacuated silica glass tubes. These compounds were investigated by X-ray powder diffraction, optical microscopy, microprobe analysis, Raman and optical microspectroscopy. A y-dependent separation into three distinct coloured crystallites with cubic perovskite-type structures is observed: (i) red-orange crystallites with composition NaxWO3 with slightly decreasing x (i.e. 0.8-0.72) with increasing nominal y, (ii) bluish solid solution of composition NaxNbyW1-yO3 and (iii) white crystallites of a new phase having defect perovskite-type structure with composition Na0.5NbO2.75. - Graphical abstract: Optical micrograph of a polished sample of nominal composition Na0.8Nb0.4W0.6O3 showing a mixture of three different coloured crystals: red, light blue and white. The scale bar is 30 μm