Structures incommensurables modulées par des défauts dans la transition de phase du bronze de titane Na 1- xTi 4O 8: Etude par diffraction électronique et microscopie électronique de haute résolution
The electron diffraction patterns of the bronze structures Na 1- xTi 4O 8, 0 < x < 0.2, are characterized from 90 to 400 K by the presence of incommensurate superstructure reflections, whose intensity and position varies with composition and temperature. These diffraction patterns are interpreted in terms of a modulation with respect to the occupation of the planes containing Na atoms: the incommensurability results from the presence in a pseudo-periodic way of planes which are either completely or partially occupied by Na atoms. The partial degree of occupation of the latter disordered planes increases with temperature, and their average periodicity varies in a quasi-continuous way. The description of the structure model in terms of a modulation by means of interfaces is confirmed from high-resolution electron micrographs. Moreover, the interpretation of the experimental images and diffraction patterns is verified by computer calculations based upon a dynamic electron diffraction theory. An explanation which does not make use of displacive modulations is useless in the case of stoichiometry of the bronze structure, since then the composition corresponds with the maximum occupation of Na atoms. The interpretation is further compared with the one generally employed for the blue bronze K 0.3MoO 3, which displays a similar behavior in certain respects.