Effect of praseodymium on the lattice dynamics and electronic structure ofthe Cu(1)-O(4) chain in Y1-xPrxBa2Cu3O7-delta

Y1-xPrxBa2Cu3O7-delta with x = 0.30 and 0.45 has been studied by emission M ossbauer spectroscopy using Co-57 dopant. Cobalt substitutes for copper pre dominantly at the Cu(1) site, and one can extract information on the local electronic and vibrational state of the Cu(1)-O(4) chain from the Mossbauer spectra. The major coordination environments observed for the 57Co dopant in YBa2Cu3O7-delta are fourfold square planar (species A), fivefold trigona l bipyramidal (species B), and fivefold square pyramidal (species C). We ha ve found that the reversible interconversion between species B and C, which was found earlier in YBa2Cu3O7-delta and Y0.9Pr0.1Ba2Cu3O7-delta, is still observed in Y0.7Pr0.3Ba2Cu3O7-delta with lower intensity, and it further d iminishes in Y0.55Pr0.45Ba2Cu3O7-delta. This tendency shows that the out of the chain vibration of the O(4) oxygens is gradually damped as one approac hes the critical Pr content where superconductivity is destroyed. This supp orts a phonon-assisted mechanism for high-T-c superconductivity in 1-2-3 co mpounds with significant contribution from the chains. We attribute the dam ping of the chain oxygen vibrations to the increase of the Cu(1)-Cu(1) dist ance along the chain with increasing Pr content. We have also concluded fro m Mossbauer isomer shifts that up to the critical Pr content where supercon ductivity disappears, the electronic density at the Cu(1) site seems to rem ain constant. This indicates that, in this region, any hybridization which takes place between Pr and sheet oxygens has no substantial influence on th e chains. [S0163-1829(99)00417-8].