Composition induced structural phase transitions in the (Ba1-xLax)2In2O5+x (0 Articolo

Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba1-xLax)2In(2)O(5+x), 0 <= x <= 0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x similar to 0.2. The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with first-order character for the tetragonal -> orthorhombic transition, while the cubic tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken.