A simple model of charge ordering is considered. It is shown explicitly that
at any deviation from half-filling (n=1/2) the system is unstable with
respect to phase separation into charge ordered regions with n=1/2 and
metallic regions with smaller electron or hole density. 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. An analysis of
this extended model allows us to determine the magnetic structure of the
phase-separated state and to reveal the characteristic features of manganites
and other substances with charge ordering.Comment: 9 pages, revte