Assessment of the La–Sr–Mn–O system

Abstract

In the La–Sr–Mn–O oxide system, the perovskite phase (La,Sr)MnO 3 and the two Ruddlesden–Popper phases, (La,Sr) 2MnO 4 and (La,Sr) 3Mn 2O 7, are known to show substitution of Sr 2+ for La 3+. All three phases show very interesting electronic and magnetic properties, such as giant magnetoresistivity (GMR) that strongly depend on the oxidation state of Mn. Using the CALPHAD approach and applying the compound energy formalism we model the La-solubility in the Ruddlesden–Popper phases in the simplest possible way, with La 3+ substituting for Sr 2+ and Mn 4+ simultaneously being reduced to Mn 3+. Due to the lack of experimental data we model no oxygen nonstoichiometry, even though they probably show both oxygen excess and deficiency. In the case of the perovskite phase more experimental data exists and we model all possible nonstoichiometries, oxygen deficiency, oxygen excess, Sr-solubility and also cation nonstoichiometry, which means that the ratio (La+Sr)/Mn can deviate from one. The model required might seem quite complex; however it is simply a combination of the models previously used to describe the LaMnO 3 and SrMnO 3 perovskites. Two interaction parameters are sufficient to model all experimental data on the (La,Sr)MnO 3 perovskites to within experimental uncertainty. In this paper we concentrate on the high temperature properties of the perovskite and do not model the low temperature structural and magnetic transitions.