DEFECT CHEMISTRY OF LA2-XSRXCUO4-DELTA - OXYGEN NONSTOICHIOMETRY AND THERMODYNAMIC STABILITY

The oxygen nonstoichiometry of La2-xSrxCuO4-delta (x = 0-0.3) was meas ured as a function of Sr content, temperature (400-1000 degrees C), an d oxygen partial pressure (P(O-2)= 1-1x10(-10) atm) using high-tempera ture gravimetry and coulometric titration. The oxygen nonstoichiometry ranges from oxygen excess to oxygen deficiency one depending on P(O-2 ) and the Sr content, x. Oxygen excess was observed for specimens with x less than 0.05. The dependence of oxygen excess nonstoichiometry on oxygen partial pressures was found to be explained by a model with in terstitial oxygen as a predominant defect. In oxygen-deficient regions , partial molar enthalpy and partial molar entropy of oxygen were calc ulated from the nonstoichiometry data. It was revealed from the variat ion in partial molar enthalpy that a strong interaction between the ox ygen and its vacancy exists in oxygen-deficient La2-xSrxCuO4-delta. Th e experimentally obtained partial molar entropy of oxygen was compared with those calculated assuming a so-called metal model, a hopping con duction model, and a narrow band conduction model, where the increase in oxygen vacancies hardly influences the carrier concentration, the h oles generated by oxidation of the specimen are trapped by Cu ions, an d the holes generated are itinerant, respectively. The variation in pa rtial molar entropy of oxygen could be explained well by either the ho pping model or the narrow band conduction model. The oxygen partial pr essures required for the decomposition of La2-xSrxCuO4-delta were also measured through the nonstoichiometry measurement. Discontinuity was observed in the dependence of oxygen partial pressures for decompositi on on Sr content between x = 0.05 and 0.10, suggesting an abrupt varia tion in the thermodynamic behavior of La2-xSrxCuO4-delta with the Sr c ontent in this region. (C) 1997 Academic Press.