Synthesis and characterization of sol-gel derived lanthanum hexaluminate powders and films

Powders and thin films of high-yield lanthanum hexaluminate (LaAl₁₁O₁₈) were prepared by a sol-gel route and compared with yields obtained by conventional hot-pressing of oxide powders. X-ray diffraction and transmission electron microscopy (TEM) were used to characterize powders and thin films deposited on TEM grids. While the solid-state kinetics of formation of LaA₁₁O₁₈ are known to be extremely sluggish, the yield of LaAl₁₁O₁₈ formed by the sol-gel route was much higher than that obtained by processing under similar conditions by solid-state reaction of elemental oxides. The development of a very fine grained microstructure at 1200 °C and a coarser, much more mature microstructure at 1450 °C, with strong texturing of the magnetoplumbite phase, was observed by TEM. Isolated grains of LaAlO₃ were present in all powders and films. Trace impurities, introduced most likely as impurities in the initial alumina sol, appear to have segregated to both the grain boundaries and to the external surfaces of grains in as-prepared films.