Diffusional transformation and structural relaxation in Y1Ba2Cu3-xNixO7-z compounds

In this paper the mechanism of phase transition and structural relaxation are elucidated by means of model experiments in the differential scanning calorimeter. Materials for this study were varied in structure and oxygen content by the introduction of Ni for Cu, through co-precipitation of oxolate gels, calcination, and sintering in ambient atmosphere. Both relaxation and phase transition are revealed by scrutinizing the heating rate dependence of heat capacity, and they seem to occur contemporaneously. Two higher order transitions are observed and are characterized by activation energies of 186 kJ/mole and 550 kJ/mole at the low and high temperatures, respectively. Perhaps the first transition is of a displacive type and occurs in anticipation of a reconstructive structural change from a fourfold chain to a sixfold sheet-like configuration. It is delineated by a change in relaxation kinetics from a localized type to a cooperative type with activation energies of 36 and 10 kJ/mole, respectively. The cooperative relaxation occurs at higher temperatures and is kinetically continuous across the reconstructive transition region. The structure characteristic exponent has a value of 0.70 for both localized and cooperative relaxation and resembles the two-dimensional relaxation often observed in depolymerized glasses. With kinetic impediment the transition region seems to approach a lambda-like behavior. In the subtransitional region, impediments in relaxation are useful in delineating oxygen stoichiometric differences between samples. The above discussion is supported by the structural and chemical characteristics that were gathered by x-ray powder diffraction, Raman spectroscopy, scanning electron microscopy and microprobe, and low temperature resistance measurements on Y123 samples containing 5 and 10 at. % Ni, respectively