New synthetic routes for preparing perovskites: Electrochemical oxidation and oxidation by NO₂

Two new synthetic low temperature routes for preparing perovskite-related materials, especially metastable phases are depicted. The first one, the electrochemical oxidation, uses the electric field as the driving force for intercalating oxygen atoms within parent oxide networks. The reaction is achieved under anodic potential, in alkaline solution (1 M KOH or NaOH), at room temperature, in air. This process has been used for preparing various perovskite compounds such as AMO₃ (A = Sr, La; M = Fe_1-xCo_x) or A₂MO_4+δ (A = La, Nd, Sr). The most relevant results are reported. Potentiostatic and galvanostatic experiments have shown that the amount of intercalated oxygen can be controlled and that the process is reversible. Structural as well as electronic aspects of the oxygen intercalation are discussed and a reaction mechanism is proposed. The second route is based on the exothermic reaction of nitrogen dioxide NO₂ with NH⁺₄ ions at moderate temperatures (typically T < 300°C), which allowed the destruction of NH⁺₄ cations in situ. Topotactic reactions are described for preparing new hexagonal forms of WO₃ or MoO₃. The reaction process is discussed.