Nanocrystalline transition aluminas: Nanostructure and features of x-ray powder diffraction patterns of low-temperatureAl2O3polymorphs

Low-temperature $\ensuremath{\gamma}$-, $\ensuremath{\eta}$-, and $\ensuremath{\chi}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ polymorphs were studied by high resolution transmission electron microscopy and x-ray powder diffraction along with computer simulation of the diffraction patterns. Planar defects lying on the {111}, {110}, and {100} planes, which are the origin of the broadening of x-ray diffraction peaks in different forms of aluminium oxide, were revealed. In addition to providing strong experimental support for the imperfect character of the specimen structures, these results demonstrate the possibility of using nanosized crystalline domains with a spinel-type arrangement, which are regularly shaped and have a specified, developed face and bounding surfaces, for the description of the nanostructure of a whole variety of low-temperature ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ derivatives. It was found that different ways of domain packing in the oxide particles with a subsequent formation of planar defects contribute to specific kinds of line-shape broadening observed in the x-ray diffraction patterns of these materials. The mechanism of the vacancy generation upon propagation of the shear-type defect in the {110} plane of the spinel structure, which changes the ${\mathrm{Al}}_{3}{\mathrm{O}}_{4}$ stoichiometry to ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, is also discussed.