Bi1-xCaxMnO3(x= 0.4 and 0.45): X-ray Single-Crystal and Electron Microscopy Study

The superstructure of Bi 1-x Ca x MnO 3 (x = 0.4 or 0.45), which exhibits a charge/orbital-ordered state at room temperature (Tco ≈ 330 K), has been successfully determined at different temperatures using single-crystal X-ray diffraction data and the superspace formalism. This peculiar approach provides a very convenient way to analyze all possible 3D symmetry (P2 1 /m, Pm, and Pnm2 1 ) of the superstructure. The structural model of the ordered state has been confirmed thanks to complementary data from several other techniques (electron diffraction, high-resolution electron microscopy (HREM), and synchrotron high-resolution X-ray powder diffraction, all vs T). It is compatible with an orthorhombic cell (a = 11.002(2) A, b = 7.588(1) A, c = 5.425(4) A at 150 K) and the space group Pnm2 1 . The structural model is based on the alternation of one double band of Mn(1)O 6 octahedra and one double band of Mn(2)O 6 octahedra, the two octahedron types exhibiting almost similar distortion. HREM images were then simulated from the refined atomic positions; the good agreement with the experimental images confirms the model. Finally, the relationships between structural characterizations and magnetic properties have been investigated for these compounds.