UNUSUAL REDOX RESPONSE OF M=NI IN YBA2(CU1-XMX)3OY

We report here on the changes in behavior, including transitions from superconducting to semiconducting, as a function of heat treatment of YBa2(Cu1-xNi(x))3O(y) with x = 0.05. In particular, conventional mater ials prepared by slow cooling from approximately 1200 K (designated OP for oxygenating preparations), show superconducting onset temperature s of T(c) approximately 75 K. This is also the case when materials are quenched from air annealing temperatures T(A) greater-than-or-equal-t o 1123 K and subsequently reoxygenated at T(a) = 673 K. However, when OP are air annealed at T(A) = 1023 K, quenched to liquid N2 (designate d K parent or K(p) in this state), and subsequently reoxygenated at T( a) = 673 K (designated as K(o) compound), no superconductivity is obse rved in the resistivities down to 4.2 K. Also, magnetic susceptibiliti es of K(o) compounds show relatively large effective magnetic moments and either no, or only trace amounts of, superconductivity. Both K(p) (quenched from T(A) = 1023 K) and K(o) were orthorhombic, while the co nventional parents (quenched from T(A) greater-than-or-equal-to 1123 K ) were tetragonal. Possible origins for this unusual K(o) phenomenolog y are discussed and comparisons are drawn with other cases where therm al treatment results in loss of superconductivity.