Assessment of the La–Sr–Mn–O system

In the La–Sr–Mn–O oxide system, the perovskite phase (La,Sr)MnO3 and the two Ruddlesden–Popper phases, (La,Sr)2MnO4 and (La,Sr)3Mn2O7, are known to show substitution of Sr2+ for La3+. 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 La3+ substituting for Sr2+ and Mn4+ simultaneously being reduced to Mn3+. 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 LaMnO3 and SrMnO3 perovskites. Two interaction parameters are sufficient to model all experimental data on the (La,Sr)MnO3 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.