SYNTHESIS, SUPERCONDUCTING PROPERTIES AND STRUCTURAL (INCLUDING ELECTRON-DIFFRACTION) STUDIES OF NA-CHI-NBO2 AND LI-CHI-NBO2

Monophasic NaxNbO2 (x = 0.5 - 0.6), T-c approximate to 4.1 K was prepa red by solid state reaction between Na3NbO4 and Nb. X-ray powder diffr action data from a monophasic sample were used for a refinement of the structure of Na0.5NbO2 (R(p) = 7.4 % and R(I) = 5.3 %), using the Rie tveld method. At low x-values a closely related second phase can occur . Rietveld refinement using X-ray powder diffraction data showed this phase to be isostructural with NaNbO2 but with the chemical formula Na 0.1NbO2. Transmission electron microscopy studies of NaxNbO2 and LixNb O2 revealed the presence of superstructures in some crystallites. The electron diffraction patterns resemble to some extent those reported f or transformations among tantalum chalcogenides.