STUDY ON THE CRYSTAL AND ELECTRONIC-STRUCTURE OF Y1-XPRXBA2CU3O7-Y CERAMICS

PrBa2Cu3O7-y (PrBCO) is unique in the RBa2Cu3O7-y (R = rare earth) ser ies because it is not superconducting. In fact, for Y1-xPrxBa2Cu3O7-y, T-c drops monotonically with Pr concentration, with T-c going to zero at 55% Pr. There have been many studies of this material with the hop e that an explanation for the lack of superconductivity in PrBCO might help explain why YBa2Cu3O7-y is superconducting. Unfortunately, to da te, none of the proposed explanations has been totally satisfactory an d without objection due to the considerable controversy of the results obtained by different experiments. In this paper, a series of samples of Y1-xPrxBa2Cu3O7-y (YPrBCO) (0 less than or equal to x less than or equal to 0.55) have been studied by x-ray diffraction, Raman spectros copy and XPS. The results reveal that the Pr dopant does not change th e orthorhombic symmetry of the samples. The peak positions at 502 and 334 cm(-1) of the Raman spectra become shifted for different Pr concen tration, which suggests a change in oxygen content upon substitution o f Pr ion for Y ion. There are two valences of Pr ions in YPrBCO. The v alence of Pr is changed with change in Pr content x. The relative cont ent of Pr3+ and Pr4+ is closely related to the total Pr content x. The valence of Pr is close to 3+ for small x, and is increased as x incre ases. The valence change of the Pr ion would directly result in the in crease of oxygen content. A ligand hole localized on the Pr-O(2)/O(3) band would remove a hole from the conduction band and yield a formal P r4+ site. The strong hybridization between Pr 4f electrons and conduct ion holes in the CuO2 planes induces a localization of mobile holes ca using the suppression of superconductivity in the YPrBCO system. In ad dition, the existence of some extra oxygen atoms around Pr wilt also a ffect the distortion of the CuO2 planes and possibly assist in the sup pression of the superconductivity.