Crystal structure and physical properties of R(Ba1−xSrx)2Cu3Oy (R=Nd, Pr) prepared under reducing atmosphere

The effects of Sr substitution for Ba on the crystal structure, oxygen content and physical properties of polycrystalline samples of R(Ba1−xSrx)2Cu3Oy (R=Nd, Pr) have been investigated. The samples were prepared by sintering at a high temperature under reducing atmosphere, followed by annealing under O2 flow. The structure parameters were refined by the Rietveld analysis of X-ray powder diffraction data. The occupation factors of each site were analyzed under the assumption that a part of Sr ions occupies the R site and pushes the same amount of R ions into the Ba site. For the R=Nd, the ratio of Sr on the Nd sites g(Sr/Nd) increases with x, from 0 for x=0 to 0.11 for x=0.5. The orthorhombicity decreases continuously with x. On the other hand, for R=Pr, the site exchange between Sr and Pr does not occur. The orthorhombicity keeps a high value until x=0.3 and drastically becomes zero at x≥0.3. The electrical resistivity and the Néel temperature TN increases with x until x=0.3, and then decreases. These results show that the high decrease rate of Tc in Nd(Ba1−xSrx)2Cu3Oy is attributed to the occupation of Ba sites by Nd and the non-superconductivity of PrBa2Cu3Oy is not due to the occupation of Ba site by Pr. It is also suggested that the hybridization of Pr 4f and O 2p π orbitals which prevents the replacement of Pr by Sr increases with x until x=0.3 and then abruptly turns to decrease.