Phase separation in systems with charge ordering

A simple model of charge ordering is considered. It is explicitly shown that at any deviation from half-filling (n not equal 1/2), the system is unstable with respect to the phase separation into the charge ordered regions with n = 1/2 and the metallic regions with a smaller electron or hole density. A possible structure of this phase-separated state (metallic droplets in a charge ordered matrix) is discussed. The model is extended to account for the strong Hund-rule onsite coupling and the weaker intersite antiferromagnetic exchange. The analysis of this extended model allows us to determine the magnetic structure of the phase-separated state and to reveal the characteristic features of the manganites and other substances with charge ordering. (C) 2001 MAIK "Nauka/Interperiodica".