Thermochemistry of YBa2Cu3-xMxOy (M = Ni, Zn)

The partial molar enthalpy of incorporation of oxygen (Delta(ox) H) and ent halpy of formation (Delta(f) H) have been measured via high-temperature rea ction calorimetry for YBa2Cu3-xMxOy (where M = Ni, Zn) in the composition r ange 0.0 < x < 0.18. Delta(ox) H and Delta(f) H values were nearly independ ent of dopant for both Ni and Zn, with enthalpies of oxidation in the range -86 to -110 kT/(mol 1/2O(2)) and enthalpies of formation in the range -110 to -118 kJ/mol. The independence of thermodynamic behavior from supercondu cting properties is consistent with an electronic mechanism for T-c suppres sion in Zn-doped YBCO superconductors. Oxygen contents, lattice parameters, and T-c measurements are also reported. (C) 2000 Elsevier Science B.V. All rights reserved.