| Institution: | 1. Department of Chemistry, Physical Chemistry, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany;2. Department of Chemistry, Physical Chemistry, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany
Jülich Centre for Neutron Science (JCNS) at MLZ, Forschungszentrum Jülich GmbH, Lichtenbergstr. 1, 85748 Garching, Germany;3. Bavarian Polymer Institute (BPI), Keylab “Electron and Optical Microscopy”, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany |
| Abstract: | Hematite (α-Fe2O3) is thermodynamically stable under ambient conditions, of vast geological importance, and widely used in applications, for example, as corrosion protection and as a pigment. It forms at elevated temperatures, whereas room-temperature reactions typically yield metastable akaganéite or ferrihydrite. The mechanistic key changes underlying this observation were explored in the present study. The entropic contribution to the prenucleation hydrolysis reaction categorically implies the presence of prenucleation clusters (PNCs) as fundamental precursors. The formation of hematite is then due to a change in the reaction mechanism above approximately 50 °C, whereby the reaction limitation towards oxolation in phase-separated clusters is overcome. A model that rationalizes the occurrence of hematite, akaganéite, and ferrihydrite based on the chemistry of olation PNCs is proposed. Supersaturation and the temperature dependence of olation and oxolation rates from monomeric precursors are irrelevant in this nonclassical mechanism. |
