Oxidation state and superconducting properties of Y1-xPrxSr2Cu2.85Re0.15O7+delta

Abstract

Superconductivity exists in orthorhombic Y1-xPrxSr2Cu2.85Re0.15O7+delta up to a critical concentration (x(cr)) of 0.65. A progressive decrease in T-c occurs as x increases from 0 to 0.65. A further increase in x leads to a tetragonal transformation and as a consequence the Ti vanishes; however, the orthorhombicity of these Sr-based compounds is much lower than that observed for the Ba analog, Y1-xPrxBa2Cu3O7-delta and, hence, the T-c. On the one hand, crystal chemistry correlations indicate that the Pr ion is in trivalent state while on the other hand, the stabilizing cation, viz., Re, is in hexavalent state which accounts for the excess oxygen (>7.0) in the system, The high x(cr) value of the Sr series compared to the Pa series (x(cr)=0.55) is attributed to the much reduced orbital overlap of the trivalent Pr(4f) state with the Cu(3d(x2-y2))-O(2p) conduction band, via hole localization and/or pair breaking, and is not due to the much discussed hole filling by tetravalent Pr. (C) 1996