Chemical Heterogeneity in a Single Phase: Bi0.15Ca0.85MnO3, a Case Example of Macroscopic Phase Segregation

Abstract

Ultrahigh-resolution synchrotron diffraction for “Bi0.15Ca0.85MnO3” at 300 K shows a unique pattern broadened by microstrains. The joint Rietveld refinement of synchrotron and neutron data at RT (room temperature) indicates that this phase is nonstoichiometric due to calcium vacancies. However, the synchrotron pattern at 10 K shows two phases one being monoclinic and the other orthorhombic. The refined weight fractions were 67.7(6) and 28.8(3)%, respectively. The joint Rietveld refinement at low temperature has established that this macroscopic phase separation/segregation is due to different thermal evolutions of the heterogeneous material. Orthorhombic and monoclinic low-temperature phases have different calcium contents. The sample at RT is formed by small domains (∼2000 Å) with minor variations in the cation stoichiometry, but it displays a single orthorhombic GdFeO3-type powder pattern. The structural transition driven by the orbital ordering unmasks these domains, and the macroscopic phase segregation comes suddenly 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 reported data for manganites should be revisited.