Assessment of the Sr-Mn-O system

A thermodynamic assessment of the Sr-Mn-O system is presented. The main practical relevance of this system is that it contains the perovskite phase SrMnO3, which is the Sr-rich end member of the phase (La,Sr)MnO3, that rinds widespread use as a cathode materia I for solid oxide fuel cells (SOFCs) and has recently attracted a lot of attention due to its interesting giant magneto-resistive properties. The thermodynamic parameters are optimized by applying the CALPHAD method. The SrMnO3-z phase exists in two modifications, a layered hexagonal modification at low temperatures and a perovskite modification at high temperatures. Both modifications show considerable oxygen deficiencies, which are modeled using the compound energy model. The sublattice occupation of the phases is (Sr2+) (Mn3+, Mn4+)(O2-, Va)(3). On reducing Mn4+ to Mn3+, oxygen vacancies are formed. The phase SrMn3O6-z, also shows an oxygen deficiency, which is modeled in an identical way. The Ruddlesden-Popper phases Sr2MnO4 and Sr3Mn2O7 and the phases Sr7Mn4O15 and Sr4Mn3O10 are modeled as stoichiometric phases. The ionic liquid is modeled using the two-sublattice model for ionic liquids. The stability and thermodynamic data on many of the phases in this system are poorly known. For this reason, some aspects of this assessment must be regarded as tentative.