Photoemission and raman studies of Y 1 − xPr xBa 2 − ySr yCu 3O 7 − δ system

Abstract

Photoemission spectra (PES) and Raman measurements have been performed for Y 1 − x Pr x Ba 2 − y Sr y Cu 3O 7 − δ 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 YBa 2Cu 3O 7 − δ) 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-totetragonal (OT) transition takes place at y = 0.75 in Y 0.4 Pr 0.6 Ba 2 − y Sr y Cu 3 O 7 − δ 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.