THE EFFECT OF CA SUBSTITUTION ON THE STRUCTURE AND THE RAMAN-ACTIVE PHONONS IN Y1-XCAXBA2CU3O7-DELTA

Micro-Raman measurements and Rietveld refinements of x-ray data have b een carried out in the superconducting system Y1-xCaxBa2Cu3O7-delta wi th 0 less than or equal to x less than or equal to 0.12, to delineate the effect of the excess doping on the Raman active phonons and especi ally 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 apic al 0(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 th at has been reported for the oxygen overdoped pure YBa2Cu3O7-delta com pound, suggesting that the softening of this mode observed in those sa mples 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 phono n increases considerably. This increase is due to the appearance of an other mode of B-1g symmetry that could correspond to a hypothetical Ca Ba2Cu3O7-delta phase separated from the pure YBCO phase.