Phase separation in Ca substituted YBCO

A room temperature micro-Raman spectroscopic study at hydrostatic pressures up to 4GPa has been carried out on Y1-xCaxBa2Cu3O7-delta polycrystalline s uperconducting compounds with x = 0.00-0.20 and delta = 0.045-0.005. Apart from the five strong Raman active phonons (4A(g) + 1B(1g)-like), additional structures have been observed in the y(cc)(y) over bar (402 cm(-1)) and th e y(xx)(y) over bar (300 and 322 cm(-1)) scattering configurations with inc reasing intensities with the Ca content. The modes that appear in the out-o f-phase B-1g phonon of O-pl are related to the disorder introduced by the C a substitution for Y and to a local mode induced by the Ca atoms. The varia tion, with x, of the relative intensities of the constituent peaks of the O -pl in-phase phonon indicates that this phonon behavior depends on the amou nt of oxygen and not the concentration of the Ca substitution introduced ca rriers. There seem to exist two oxygen content differentiated phases which are present also in the pure oxygen overdoped YBCO. The insensitivity of th e O-pl in-phase mode frequency combined with the behavior of the O-apex pho non, can be attributed to the excess charge distribution away from the CuO2 planes. A phase separation seems to be induced for hydrostatic pressures a bove 3GPa possibly related to the Ca atom cluster formation.