J. Phys.: Condens. Matter 10 (1998) 7015–7024. Printed in the UK PII: S0953-8984(98)91483-7 Study on the crystal and electronic structure of Y 1−x Pr x Ba 2 Cu 3 O 7−y ceramics Shi Lei†, Huang Yunsong†, Jia Yunbo†, Liu Xianming†, Zhou Guien† and Zhang Yuheng†‡ † Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ‡ CCAST, World Laboratory, Beijing 100080, People’s Republic of China Received 10 February 1998 Abstract. PrBa 2 Cu 3 O 7−y (PrBCO) is unique in the RBa 2 Cu 3 O 7−y (R = rare earth) series because it is not superconducting. In fact, for Y 1−x Pr x Ba 2 Cu 3 O 7−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 hope that an explanation for the lack of superconductivity in PrBCO might help explain why YBa 2 Cu 3 O 7−y is superconducting. Unfortunately, to date, none of the proposed explanations has been totally satisfactory and without objection due to the considerable controversy of the results obtained by different experiments. In this paper, a series of samples of Y 1−x Pr x Ba 2 Cu 3 O 7−y (YPrBCO) (0  x  0.55) have been studied by x-ray diffraction, Raman spectroscopy and XPS. The results reveal that the Pr dopant does not change the orthorhombic symmetry of the samples. The peak positions at 502 and 334 cm −1 of the Raman spectra become shifted for different Pr concentration, which suggests a change in oxygen content upon substitution of Pr ion for Y ion. There are two valences of Pr ions in YPrBCO. The valence of Pr is changed with change in Pr content x. The relative content of Pr 3+ and Pr 4+ 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 increases. The valence change of the Pr ion would directly result in the increase 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 Pr 4+ site. The strong hybridization between Pr 4f electrons and conduction holes in the CuO 2 planes induces a localization of mobile holes causing the suppression of superconductivity in the YPrBCO system. In addition, the existence of some extra oxygen atoms around Pr will also affect the distortion of the CuO 2 planes and possibly assist in the suppression of the superconductivity. 1. Introduction It is well known that different rare-earth ions (except Pr, Ce and Tb) can be substituted in place of yttrium ions in YBa 2 Cu 3 O 7−y (YBCO), giving superconductivity with nearly the same critical temperature T c = 90 K. Ce and Tb yield the stable perovskites BaCeO 3 and BaTbO 3 rather than the YBa 2 Cu 3 O 7 -type structure. The Pr element is the only one which is isostructural with YBCO but is not a high-T c superconductor. As the Pr concentration is increased in Y 1−x Pr x Ba 2 Cu 3 O 7−y (YPrBCO) T c is monotonically decreasing, with superconductivity vanishing at a concentration of x ≈ 0.55 [1–3]. The normal-state electrical behaviour also undergoes a transition from metallic (for x  0.55) to semiconducting (for x  0.55) at roughly the same concentration. There have been many studies of this material with the hope that an explanation for the lack of superconductivity in YPrBCO might help explain why YBCO is superconducting. The mechanisms proposed 0953-8984/98/317015+10$19.50 c  1998 IOP Publishing Ltd 7015 Note: family name followed by the given name, such as "Huang Yunsong", is the Chinese naming convention. However, the standard naming convention in the literature is given name followed by family name. Therefore, this paper is often cited as ... by S. Lei, H. Yunsong, et al.