Chemical heterogeneity in a single phase: Bi0.15Ca0.85MnO3, a case exampleof macroscopic phase segregation

Ultrahigh-resolution synchrotron diffraction for "Bi0.15Ca0.85MnO3" at 300 K shows a unique pattern broadened by microstrains. The joint Rietveld refi nement of synchrotron and neutron data at RT (room temperature) indicates t hat this phase is nonstoichiometric due to calcium vacancies. However, the synchrotron pattern at 10 K shows two phases one being monoclinic and the o ther orthorhombic. The refined weight fractions were 67.7(6) and 28.(3)%, r espectively. The joint Rietveld refinement at low temperature has establish ed, that this macroscopic phase separation/segregation is due to different thermal evolutions of the heterogeneous material. Orthorhombic and monoclin ic low-temperature phases have different calcium contents. The sample at RT is formed by small domains (similar to 2000 Angstrom) with minor variation s in the cation stoichiometry, but it displays st single orthorhombic GdFeO 3-type powder pattern. The structural transition driven by the orbital orde ring unmasks these domains, and the macroscopic phase segregation comes sud denly into sight. The associated structural changes and magnetic structures are reported. A unique pattern above the orbital ordering temperature (RT in most cases) is not any longer a proof of chemical homogeneity, or single phase existence, in manganites. So, the discussion of many previously repo rted data for manganites should be revisited.