THE YBA2CU3OZ PHASE-DIAGRAM

The variations of the Seebeck coefficient, S, for YBa2Cu3Ox as a funct ion of temperature at constant oxygen composition are reported for 6.4 less than or equal to z less than or equal to 6.9 and 40 less than or equal to T/degrees C less than or equal to 450. These curves, S(T)(x) , exhibit two kinds of kinks: at low temperature, 100 less than or equ al to T-a less than or equal to 110, for 6.479 less than or equal to z less than or equal to 6.675 and at high temperature, 190 less than or equal to T-b less than or equal to 375, for 6.4 less than or equal to z less than or equal to 6.9. It is likely that the first kink corresp onds to the O-I-O-II phase transition as indicated by a recent synchro tron X-ray diffraction study. Taking into account that the orthorhombi c-tetragonal phase transition is similarly marked by an S(T)(x) kink, one concludes that the high-temperature T-b kink likewise corresponds to a phase transition, the nature of which is unknown.