COLLOIDAL EQUILIBRIUM IN INSULATING CRYSTALS - POSSIBLE APPLICATION TO RELAXOR FERROELECTRICS

Decomposition in a multicomponent insulating compound with a considera ble Coulomb interaction between ions is considered. A specific situati on where decomposition occurs due to redistribution of ions of the sam e sign (either cations or anions) whereas all other atoms form an immo bile frame is discussed. Such a situation may occur if a diffusivity o f the ions of the same sign is much higher than that of atoms comprisi ng the frame. It is shown that under these conditions the decompositio n results in solid-state colloidal equilibrium. The colloidal state is formed by an array of coherent precipitate phase particles imbedded i nto a parent phase matrix. These particles are of the same stable size . This size, as well the volume fraction of a precipitate phase and th e equilibrium composiitons of precipitate and matrix phases, are deter mined by the balance between electrostatic and interfacial energies. T he balance proves to be dependent on overall stoichiometry. This situa tion drastically differs from the conventional situation where the com positions of coexisting equilibrium phases do not depend on overall st oichiometry of a system (the lever rule is valid) and where the coarse ning process always occurs. The proposed model is applied to relaxor f erroelectrics to explain occurrence of a mesoscopic inhomogeneous stru cture that does not undergo coarsening.