INFLUENCE OF CU-SITE SUBSTITUTION ON THE STRUCTURE AND SUPERCONDUCTING PROPERTIES OF THE NDBA2CU3-XMXO7+DELTA (M=FE,CO) AND NDBA2CU3-XMXO7-DELTA (M=NI,ZN) SYSTEMS

The effect of substitution at the Cu site by the transition elements Fe, Co, Ni, and Zn on the structure and superconducting properties of NdBa2Cu3O7 has been investigated. Compared to the YBa2Cu3-xMxO7+delta (M = Fe, Co, Ni, Zn) system, considerably high solid solubility and drastic T(c) suppression is achieved in the Nd 1:2:3 system. The maximum solid solubilities achieved for the dopants are x = 1.0 for M = Fe and Co and x = 0.8 for M = Ni and Zn. An orthorhombic-to-tetragonal (O --> T) transition is observed at x = 0.08 for M = Fe, Co similar to the case of the M-doped Y 1:2:3 system. However, a second T --> O transition is observed in Nd 1:2:3 at x = 0.5 for M = Co and x = 0.8 for M = Fe. For the Ni- and Zn-doped phases, an O --> T transition is observed at x = 0.4 for M = Ni and x = 0.5 for M = Zn. The doped phases are superconducting up to x = 0.2 (M = Fe, Co, Ni) and x = 0.08 (M = Zn). The initial rate of T(c) suppression (up to 3.33 at. %) is approximately 14 K/at. % for M = Fe, Co, and Ni and approximately 28 K/at. % for M = Zn and is much higher than that reported for the Y 1:2:3 system. The Fe- and Zn-doped phases, heated in flowing N2 gas at 800-degrees-C followed by reoxygenation at 450-degrees-C show higher T(c) and sharper superconducting transition. Hole doping by Ca substitution (y) at the Nd site in the Zn-doped (x = 0.1) semimetallic-semiconducting Nd 1:2:3 phase induces metallicity and superconductivity for y greater-than-or-equal-to 0.1 and a T(c) of 32 K is obtained for y = 0. 2.