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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 (T CO ≈ 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) Å, b ) 7.588(1) Å, c ) 5.425(4) Å 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.

Measurements of the electrical and thermal conductivities, thermopower, and paramagnetic susceptibility have been performed on single-crystal samples of Bi1/2Sr1/2MnO3, complemented with the x-ray powder diffraction data. A pronounced hysteretic behavior, observed below the cubic-to-orthorhombic transition at Tcrit=535 K, is related to the onset of long-range charge order (CO) at TCO=450 K and its further evolution down to about 330 K. The diffraction data suggest that the charge-ordered state is formed by Zener pairs, represented by Mn4+ dimers linked by one extra eg electron, and is possibly stabilized by cooperative Bi,Sr displacements. An extremely low thermal conductivity is observed down to the lowest temperatures, without any recovery at the antiferromagnetic ordering temperature TN=150 K. Such behavior points to a presence of strong scatterers of phonons. Their possible origin can be linked to “optical-like” oscillations that are associated with fluctuating charges within the Zener pairs.

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