Local structural distortions and phase separation in cuprates

We report evidence from Raman measurements of local lattice distortions in the cuprates, which are induced at room temperature by varying the hydrostatic pressure and correlate with changes in the superconducting transition temperature. Anomalous nonlinear pressure dependence is observed for almost all A(g) phonons of the YBa2Cu3O6.5, YBa2Cu3Oover, and YBa2Cu4O8 superconducting single crystals, which occurs at pressures where the transition temperature shows also characteristic changes. The results are compared with a similar correlation found between transition temperature and spectral modifications in Bi2Sr2CaCu2O8 by pressure and in YBa2Cu3Ox and La2-xSrxCuO4 by chemical doping. There are strong indications that the saturation of T-c with pressure or compositional doping in these cuprates is mainly related with lattice instabilities and probably also with phase separation phenomena.