Phase separation in fully oxygenated Y1-yCayBa2Cu3Ox compounds -: art. no. 014303

A micro-Raman spectroscopic study has been carried out on fully oxygenated Y1-yCayBa2Cu3Ox (YBCO) compounds with y=0.0-0.2 and oxygen concentration varying with Ca in the range (xsimilar to6.960 and 6.995). The spectral profiles of the 4A(g) and 1B(1g)-like symmetry phonons show minor modifications with Ca, but two modes that appear close in energy to the in-phase and the out-of-phase phonons gain intensity with the Ca substitution. The changes in the intensity of these two satellite modes with the amount of Ca are correlated with modifications in the dimpling of the CuO2 planes, as detected by extended x-ray absorption fine structure, and can be attributed to the formation of superstructures and to phase separation phenomena. The evolution of two bands in the in-phase phonon, associated in pure YBCO with local structural distortions and the coexistence of optimal and oxygen overdoped phases, remains completely independent of the Ca concentration. It appears that the Ca atoms have the tendency to be surrounded by Y atoms forming an independent nanophase of the type Y0.5Ca0.5Ba2Cu3Ox with the additional carriers from Ca not contributing to an excess doping of the system.