Metastable mixed oxide phase synthesis

Preparation of Metastable Mixed Oxide Phase. The key step in the synthesis of the LTM phase is the preparation of the metastable mixed oxide phase. This mixed oxide phase is prepared by calcination of the LDH precursor. At temperatures between initial LDH decomposition and spinel phase formation, a series of metastable phases, both crystalline and amorphous, can be formed 18). The properties of these phases depend on the cations constituting the original LDH, preparation and thermal decomposition... 

Mixed oxides have the ability to form paracrystalline phases that represent an intermediate phase between the crystalline and the amorphous phase. Because of the disordered structure paracrystallinity causes defects that occur due to the inhibition of recrystallization and are initiated by the introduction of additional anions or molecules into the lattice structure. The formation of these deformed crystalline structures depends on the used synthesis method as well as the nature of the cation precursor. Thermal decomposition of the LDHs leads to the formation of the metastable phase that contains divalent and trivalent cations with densely packed configuration [2,32]. After the reduction, tri-valent ions are trapped inside the crystallites of divalent ions causing the formation of active centers. The centers that exhibit higher reactivity are believed to be formed on the surface of the paracrystalline crystals. After deactivation, the grain size of these mixed oxides increases due to the gradual removal of the defects in the crystal lattice [2]. 

Since in most cases the mixed layered compounds are metastable phase, they are not easy to be synthesized by using a normal method. Soft chemical method is a powerful method for the synthesis of such metastable phases. Some mixed layered compounds have been synthesized by insertion of metal hydroxides into clay compounds with cation-exchange and pH titration methods, and it has been also found that these mixed layered compounds exhibit characteristics differing from the metal hydroxides and the clay precursors [15-17]. However, only a few studies have been reported on the synthesis of mixed layered metal oxides. Recently, we have synthesized a mixed layered manganese... 

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