Role of Point Defects on the Properties of Manganites

Abstract

La1-xCaxMnO3+delta and La1-xNaxMnO3+delta samples with well defined cation molecularity and oxygen contents are analyzed with XRPD, electrical conductivity, electron paramagnetic resonance and static magnetization measurements. Point defects are introduced in the structure by: i) aliovalent cation doping (substitutional defects), ii) oxygen over-stoichiometry (cation vacancies) and iii) oxygen under-stoichiometry (oxygen vacancies). The cation doping mainly influences the material by affecting the tolerance factor and the oxidation of the Mn ions; the cation vacancies affect the magnetic properties by interrupting the interaction paths between Mn4+-Mn3+ ions whereas the oxygen vacancies have a stronger influence on the structural and electrical properties and act on the magnetic properties by the overall decrease on the Mn4+ amount and by the creation of sample regions with different magnetic features. We can state that the basic magnetic and electrical behaviors of the Ca- and Na-doped lanthanum manganite compounds are driven from the Mn4+-Mn3+ ratio and all the peculiar behaviors are indeed caused by defects, concentration gradient and, more generally, by lattice disorder.