Superconductivity and crystalline structure in the Na-W-O system

X-ray diffraction analysis, using the Rietveld refinement method, of samples of the Na_xWO₃ system for 0.2 ≤x≤0.4, prepared at 900°C, shows that for x≥0.28 the samples are a mixture of two phases: one with tetragonal crystalline structure and space group P4/mbm, and the other with cubic crystalline structure and space group Pm3m, with the last one as the majority phase. This result must be taken into account in the models that try to explain the observed variation of the transition temperature to the superconducting state with sodium concentration in this system. In the tetragonal phase, only the sites surrounded by ten WO₆ octahedra are occupied by sodium atoms.