OXIDATION-STATE AND SUPERCONDUCTING PROPERTIES OF Y1-XPRXSR2CU2.85RE0.15O7+DELTA

Superconductivity exists in orthorhombic Y1-xPrxSr2Cu2.85Re0.15O7+delt a up to a critical concentration (x(cr)) of 0.65. A progressive decrea se 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 van ishes; however, the orthorhombicity of these Sr-based compounds is muc h lower than that observed for the Ba analog, Y1-xPrxBa2Cu3O7-delta an d, hence, the T-c. On the one hand, crystal chemistry correlations ind icate that the Pr ion is in trivalent state while on the other hand, t he 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 t he Sr series compared to the Pa series (x(cr)=0.55) is attributed to t he 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 pai r breaking, and is not due to the much discussed hole filling by tetra valent Pr. (C) 1996 American Institute of Physics.