Oxidation state and superconducting properties of Y_1-xPr_xSr₂Cu_2.85Re_0.15O_7+δ

Abstract

Superconductivity exists in orthorhombic Y_1-xPr_xSr₂Cu_2.85Re_0.15O_7+δ 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 Tc vanishes; however, the orthorhombicity of these Sr-based compounds is much lower than that observed for the Ba analog, Y_1-xPr_xBa₂Cu₃O_7-δ 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 Ba 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.