Oxidation kinetics in oxygen deficient YBa/sub 2/Cu/sub 3/O/sub 7-x/ studied by neutron powder diffraction

A high-resolution, multidetector neutron powder diffractometer has been constructed and used for online studies of the oxidation kinetics in ceramic powders of oxygen-deficient YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBACUO). The structural phase transition between ordered orthorhombic and disordered tetragonal phase has been studied in pure oxygen and under vacuum. A continuous transition at 655 degrees C is observed in a pure oxygen atmosphere under equilibrium conditions, whereas an irreversible transition is observed at 470 degrees C under vacuum. Reduced YBACUO corresponding to x approximately=1.0 has been obtained by evacuation at 705 degrees C and has been shown to be stable in air. The oxidation processes of reduced YBACUO in a pure oxygen atmosphere have been studied using a semidynamical stepwise increase in temperature and under isothermal conditions. The results show that the in-diffusion under oxidation of reduced YBACUO is more easily accomplished than the corresponding out-diffusion process under reduction. From the structural data it is concluded that oxidation takes place in a mixed-phase medium of coexisting tetragonal and orthorhombic phases.<
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