Thermodynamic studies of the orthorhombic--tetragonal transition in Ba/sub 2/YCu/sub 3/O/sub 7-//sub delta/

The superconducting oxide Ba/sub 2/YCu/sub 3/O/sub 7-//sub delta/ is known to exist in two crystallographic structures, being orthorhombic or, at higher temperature, tetragonal. Thermogravimetric experiments have been performed to study their transition at different oxygen partial pressures. Weight loss curves, recorded by heating samples at various constant rates, each exhibit two inflection points. In our interpretation, such points delimit the regime within which the two structural phases coexist. Limiting values of temperature at these points, calculated by extrapolation to zero heating rates, and the corresponding magnitudes of oxygen nonstoichiometry are presented. Such data have basic significance as well as relevance to the thermal processing of materials.