On the effect of heterovalent substitutions in ruthenocuprates

We discuss the properties of superconducting derivatives of the RuSr2GdCu2O8 (1212-type) ruthenocuprate, for which heterovalent doping has been achieved through partial substitution of Cu ions into the RuO2 planes (Ru1-xSr2GdCu2+xO8-delta, 0 less than or equal to x less than or equal to 0.75, T-c(max) = 72 K for x = 0.3-0.4) and Ce ions into the Gd sites (RuSr2Gd1-yCeyCu2O8, 0 less than or equal to y less than or equal to 0.1). The measurements of XANES, thermopower, and magnetization under external pressure reveal an underdoped character of all compounds. Muon spin rotation experiments indicate the presence of magnetic order at low temperatures (T-m = 14-2 K for x = 0.1-0.4). Properties of these two series lead us to the qualitative phase diagram for differently doped 1212-type ruthenocuprates. The difference in temperature of magnetic ordering found for superconducting and non-superconducting RuSr2GdCu2O8 is discussed in the context of the properties of substituted compounds. The high pressure oxygen conditions required for synthesis of Ru1-xSr2RECu2+xO8-delta, have been extended to synthesis of a Ru1-xSr2Eu2-yCeyCu2+xO10-delta series. The Cu --> Ru doping achieved in these phases is found to decrease the temperature for magnetic ordering as well the volume fraction of the magnetic phase. (C) 2002 Elsevier Science B.V. All rights reserved.