Phase Separation in Cuprates Induced by Doping, Hydrostatic Pressure or Atomic Substitution Symmetry and Heterogeneity in High Temperature Superconductors

Results of systematic Raman studies of phase separation induced on cuprates by doping, hydrostatic pressure, and atomic substitutions are presented. In YBa 2 Cu 3 O 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 2 Cu 3 O 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 2 Cu 3 O x , YBa 2 Cu 4 O 8 , and the Bi 2 Sr 2 CaCu 2 O 8 superconductors. A coexistence of phases seems to appear at a critical pressure where changes in the superconducting transition temperature have been observed.