Anisotropic X-ray peak broadening and twin formation in hematite derived from natural and synthetic goethite

Hematite obtained by dehydration of goethite at temperatures between 250 °C and 1000 °C was studied by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Starting materials were goethite mineral and synthetic goethite, respectively. Hematite derived from natural goethite shows narrow X-ray diffraction peak widths with little variation between the reflections and marginal dependence on dehydration temperature. TEM imaging reveals the existence of large (>200 nm) hematite twin domains, which is attributed to slow dehydration kinetics associated with the formation of few hematite nuclei.In contrast, strong anisotropic X-ray diffraction peak broadening is observed for hematite obtained from synthetic needle-shaped goethite at low dehydration temperatures (T < 500 °C); the peak widths significantly decrease with increasing dehydration temperature. Anisotropic peak broadening is observed only for reflections, which structure factors are dominated by the iron sub-lattice and which are not common to both twin variants of hematite. In hematite producing the strong anisotropic X-ray peak broadening extremely small twin domains with sizes ranging from 5 nm to 10 nm could be imaged by high-resolution TEM. Further, DSC and TEM observations indicate that dehydration kinetics in coarse-grained natural goethite and fine-grained synthetic goethite differ considerably. It is concluded that during the dehydration reaction taking place at the large surface area of synthetic goethite crystals hematite nuclei with ambient orientation are rapidly formed, thereby creating a finely twinned dehydration product. The experimental results prove that XRD peak broadening is mainly caused by fine twinning.