Phase separation in high-temperature superconductors and related magnetic systems

A review is given of theoretical and experimental data on spontaneous phase separation in nonsuperconducting degenerate magnetic semiconductors and related cuprate high-temperature superconductors. The following phenomena are considered: (1) The electronic phase separation occuring at frozen impurity positions as a result of charge carrier concentration in regions with a changed magnetic state; (2) impuriry (chemical) phase separation when a nonuniform impurity distribution over a crystal is driven simultaneously by interaction between impurity atoms and by their tendency to concentrate inside regions with a changed magnetic ordering.