  We relate the phase separation observed in many crystals with pronounced
electron correlations to the regions of negative electron compressibility. They
were found in several models describing strong electron correlations. At low
temperatures, these regions arise near chemical potentials corresponding to the
change of the ground state in the site Hamiltonian. The negative electron
compressibility leads to the separation of the system into electron-rich and
electron-poor domains. The energy released in the course of this separation is
absorbed by phonons. Another role of phonons is to give a definite form --
stripes or checkerboards -- to lattice distortions and domains of different
electron concentrations. The shape, direction, and periodicity of such textures
are determined by wave vectors of lattice distortions, which most strongly
scatter electrons.
