Anisotropic X-ray diffraction peak broadening and twinning in diaspore-derived corundum

Corundum produced by dehydration of diaspore at temperatures between 400 and 1000 °C was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The broadening of XRD peaks and the size of corundum twin domains are related to the dehydration temperature. At low dehydration temperatures (<450 °C) as well as at high temperatures (>700 °C) large twin domains are obtained which yield sharp XRD peaks for all reflections. Dehydration between 450 and 600 °C leads to twin domains smaller than 10 nm. In this temperature range, significant peak broadening is observed, however, only for reflections, which structure factors are dominated by the aluminum sublattice and which are not common to both twin variants of corundum. The experimental results show that XRD peak broadening is only caused by fine twinning. Porosity in corundum as a reason for XRD peak broadening is excluded: the lamellar pore system is well ordered in corundum produced at low temperatures, however narrow peaks are observed. The reasons for the development of twin domains with differing sizes at different dehydration temperatures are discussed in detail.