Phase Transitions in Sodium Tungsten Bronzes

The reported structures of tungsten bronzes include cubic, pseudocubic, tetragonal, orthorhombic, and monoclinic distortions of the perovskitetype structure; well-characterized tetragonal and hexagonal structures, as found in the series KxWO3; and some unit cells which are reported for isolated phases. This work surveys some of the phases found for various alkali tungsten bronzes. In the sodium bronzes there is a progression of distorted perovskites from x = 0 to x ≅ 0.15, in the same order as the thermal transitions of WO3 (monoclinic → orthorhombic → tetragonal). High-temperature x-ray diffractometry shows these phases to go through the same thermal transitions as WO3, except that two-phase regions intervene. Between x = 0.15 and 0.28, two tetragonal phases coexist, the second having a range of homogeneity from x = 0.28 to 0.38. This is followed by a cubic (or pseudocubic) range from x = 0.43 to 0.95.