Photoemission and Raman studies of Y1-xPrxBa2-ySryCu3O7-delta system

Photoemission spectra (PES) and Raman measurements have been performed for Y1-xPrxBa2-xSrxCu3O7-delta polycrystalline samples, especially for the x = 0.6 series. It is shown that Y3d, Ba3d, Sr3d and Ols electron binding energies shift towards lower values with increasing Sr concentration y; Cu2p core level has no apparent changes; Pr3d (obtained by subtraction of Pr3d + Cu2p from the Cu2p of YBa2Cu3O7-delta) level tends to go up with increasing Sr content. The core level results imply that Sr-doping enhances the Ba/Sr-O covalency but weakens the Pr-O covalent mixing. The valence band spectra display regular changes upon Sr-doping. We have discussed them in terms of the Pr-O covalent bonding effect, which may account for the puzzled 'ion-size effect' and probably plays an important role in suppression of superconductivity by Pr. Raman measurement shows an extra peak at 575 cm(-1) while the orthorhombic-to-tetragonal (O-T) transition takes place at y = 0.75 in Y0.4Pr0.6Ba2-ySryCu3O7-delta system, suggesting that the O-T transition be originated from the oxygen disorder in Cu(1)O layer. In addition, the out-of-phase O(2,3) bending mode is softened with increasing Sr-doping. This fact suggests that the chemical bonding strength of Y/Pr-O decreases, which is consistent with the result of PES. (C) 1997 Elsevier Science Ltd.