Oxygen nonstoichiometry and defect equilibrium in electron doped Ca0.6−уSr0.4LaуMnO3−δ

Abstract

The perovskite-like electron doped manganites Ca0.6−уSr0.4LaуMnO3−δ (0⩽y⩽0.15) were synthesized in order to study oxygen content variations with composition, temperature and oxygen partial pressure. The samples are characterized with orthorhombic Pnma structure at 0<у<0.1 and tetragonal I 4/mcm structure at у⩾0.13 according to room temperature powder X-ray diffraction (XRD). The high-temperature data of XRD in air show transition to cubic Pm3¯m structure at heating above 550°С. The variations of oxygen content in Ca0.6−уSr0.4LaуMnO3−δ with temperature and oxygen pressure were measured using thermogravimetry and coulometric titration at temperatures between 650 and 950°С in the oxygen pressure range 10−4–0.7atm. The oxygen nonstoichiometry δ changes in Ca0.6−уSr0.4LaуMnO3−δ are observed to considerably decrease with the increase in lanthanum content. In order to explain the nonstoichiometry data, a thermodynamic model is developed based on defect formation reactions 2Mn3++VO+½O2=2Mn4++O2− and 2Mn4+=Mn3++Mn5+. The respective equilibrium constants, defect formation enthalpies and entropies are obtained. The concentrations of different defects are calculated as functions of temperature, oxygen pressure and lanthanum content. The high-temperature conductivity measurements are carried out for independent verification of the suggested model.