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Sodium insertion in W18O49proceeds through the formation of a solid solution for the whole range of intercalant NaxW18O49with 0≤x≤1.8. The different features detected in −∂x/∂Evs Eplots have been associated with continuous phase transitions. In order to know more about these transitions, several samples NaxW18O49with compositions around the minimum in the −∂x/∂Ecurve have been synthesized and then characterized by both X-ray and electron diffraction techniques. An interesting correlation with the electrochemical results has been established.

A microstructural study of the perovskite La_0.5Li_0.5TiO₃ is presented. A new, tetragonal cell, a = 2^1/2d_p and c = 2a_p, is proposed on the basis of the cubic perovskite subcell (a_p ~ 3.8 Å). By transmission electron microscopy a microdomain twinned structure has been observed. The twinning corresponds to the three possible orientations of the doubled c-axis. Besides these twinned domains, other domains have also been detected where fringe spacings corresponding to 2a_p × 2a_p are present. This seems to indicate the alternation of lanthanum and lithium along the [001] direction. Copyright 1995, 1999 Academic Press