Results of systematic Raman studies of phase separation induced on cuprates by doping, hydrostatic pressure, and atomic substitutions are presented. In YBa_2Cu_3O_x, oxygen doping induces a separation into microphases from the ordering of the chains, at relative amounts that vary with concentration. In the overdoped region, a similar coexistence of phases appears, which is due to the modification of the CuO_2 buckling. The excess doping of YBa_2Cu_3O_(7-δ) by Ca creates domains of pure Y123 and others where the Ca atoms are surrounded by yttrium as first neighbours. Hydrostatic pressure induces non-linear effects in almost all A_g-symmetry phonons of YBa_2Cu_3O_x, YBa_2Cu_4O_8, and the Bi_2Sr_2CaCu_2O_8 superconductors. A coexistence of phases seems to appear at a critical pressure where changes in the superconducting transition temperature have been observed.
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