Phase Separation in Cuprates Induced by Doping, Hydrostatic Pressure or Atomic Substitution

Results of systematic Raman studies of phase separation induced on cuprates by doping, hydrostatic pressure, and atomic substitutions are presented. In YBa₂Cu₃O_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 CuO₂ buckling. The excess doping of YBa₂Cu₃O_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 YBa₂Cu₃O_x, YBa₂Cu₄O₈, and the Bi₂Sr₂CaCu₂O₈ superconductors. A coexistence of phases seems to appear at a critical pressure where changes in the superconducting transition temperature have been observed.