The effect of Ca substitution on the structure and the Raman active phonons in Y_1-xCa_xBa₂Cu₃O_7-δ

Micro-Raman measurements and Rietveld refinements of x-ray data have been carried out in the superconducting system Y_1-xCa_xBa₂Cu₃O_7-δ with 0 ≤ x ≤ 0.12, to delineate the effect of the excess doping on the Raman active phonons and especially on the characteristics of the plane oxygen modes. The increase of the Cu(2)-O(4) bond length with increasing Ca concentration induces a much smaller decrease in the frequency of the A_g phonon of the apical O(4) atom than the rare earth substitutions for Y. The frequency of the in-phase A_g vibrations of the plane oxygens decreases slightly with increasing Ca content, but remains well above the lowest value that has been reported for the oxygen overdoped pure YBa₂Cu₃O_7-δ compound, suggesting that the softening of this mode observed in those samples may not be attributed to the addition of carriers. The frequency of the out-of-phase B_1g-like vibrations of the plane oxygens is not affected by the substitution, but the width of the corresponding phonon increases considerably. This increase is due to the appearance of another mode of B_1g symmetry that could correspond to a hypothetical CaBa₂Cu₃O_7-δ phase separated from the pure YBCO phase.