The YBa2Cu3Oz phase diagram

Abstract

The variations of the Seebeck coefficient, S, for YBa₂Cu₃O_z as a function of temperature at constant oxygen composition are reported for 6.4 ⩽z⩽ 6.9 and 40 ⩽T/°C ⩽ 450. These curves, S(T)_z, exhibit two kinds of kinks: at low temperature, 100 ⩽ T_a⩽ 110, for 6.479 ⩽z⩽ 6.675 and at high temperature, 190 ⩽T_b⩽ 375, for 6.4 ⩽z⩽ 6.9. It is likely that the first kink corresponds to the O^I–O^II phase transition as indicated by a recent synchrotron X-ray diffraction study. Taking into account that the orthorhombic-tetragonal phase transition is similarly marked by an S(T)_z kink, one concludes that the high-temperature T_b kink likewise corresponds to a phase transition, the nature of which is unknown.