State of manganese ions in the structure of corundum synthesized in supercritical water fluid

The kinetics and formation mechanism of doped corundum (α-Al₂O₃) from hydrargillite (γ-Al(OH)₃) in supercritical water fluid (SCWF) in the presence of manganese ions are studied. It was ascertained that due to the reversible dehydroxylation in an aqueous medium, solid-phase transformation of hydrargillite into boehmite (γ-AlOOH) and then into corundum occurs with the formation of well-faceted corundum micro-crystals that are uniformly doped with manganese. It was found that when Mn²⁺ or MnO₄⁻ ions are introduced into the reaction medium, Mn⁵⁺, Mn⁴⁺, Mn³⁺, and Mn²⁺ ions are observed in the synthesized corundum. Meanwhile, the manganese ions form a complex defect in the corundum structure, which comprises oxygen vacancies and hydroxyl groups. The defects in corundum that emerge upon doping with manganese in SCWF are different from those in corundum doped during high-temperature synthesis.