Mechanisms of phase separation in gel-based synthesis of multicomponent metal oxides

The principles underlying metal oxide phase formation from multicomponent molecular gels are reviewed. The critical phase separation mechanisms operating at each stage of the gel process, viz. gel synthesis, gel thermolysis and oxide crystallization, are described with examples from the synthesis literature on aluminosilicates, cuprates and lead-based perovskites. It is demonstrated that direct crystallization of the equilibrium metal oxide requires synthesizing a cation-homogeneous gel, avoiding phase separation during thermolysis, and providing a low energy barrier for nucleation of the equilibrium phase. The influence of synthesis parameters and heating conditions on chemical phase separation are explained and guidelines for regulating the direct formation of metal oxides are outlined.