POINT-DEFECTS, THERMODYNAMIC AND SUPERCONDUCTING PROPERTIES OF THE NONSTOICHIOMETRIC HGBA2CUO4+DELTA PHASE

The thermodynamic properties of Hg1201 have been studied by thermograv imetry in terms of point defects, related to the departure from stoich iometry delta. The oxygen intercalation/disintercalation process is sh own to be not completely reversible in the temperature range of 150-45 0 degrees C. The reversible fraction delta depends on the oxygen pres sure according to a law in (pO(2))(1/6) which corresponds to the inser tion of interstitial oxygen and the formation of two holes per oxygen. The formation enthalpy of these defects, 0.5 eV, is rather small. A m odel based on temperature dependent barrier heights is introduced to a ccount for the non-reversible fraction. The barrier mechanism may be r elated to exchange interactions between interstitial oxygens mediated by the mercury sublattice. The thermogravimetric analysis has allowed to derive the T-c versus delta curve without resorting in particular i n the overdoped regime to high oxygen pressures.