Bulg. J. Phys. vol.34 no.s1 (2007), pp. 267-300

On the Structural Peculiarities of a Family of Half Hole Doped Bismuth Manganites

K.A. Krezhov
Institute for Nuclear Research and Nuclear Energy, 1784 Sofia, Bulgaria
Abstract. Colossal magnetoresistance (CMR), orbital ordering (OO), charge ordering (CO) and electronic phase separation are manifestations of the strong interplay of charge, orbital and lattice degrees of freedom in rare earth manganites. After a short introduction to these phenomena we present neutron diffraction results for the crystal and magnetic structures in the bismuth-based manganites Bi0.25RE0.25Ca0.5MnO3, where RE stands for La, Ho, Nd, Tm, and Bi0.5Ca0.5Mn1-xFexO3. The crystal structures make these oxides related to the large family of perovskites. In the parent compound Bi0.5Ca0.5MnO3 at room temperature the 3d eg orbitals of manganese ions order in the ac plane (Pnma setting) in an alternating fashion. We discuss the orbital order stability with respect to temperature and chemical doping. The `chemical pressure effects' i.e. structural distortions induced by changing <rA>, the average size of perovskite A-site cations, are analysed for the family Bi0.25RE0.25Ca0.5MnO3 wherein at certain temperature thermal fluctuations destroy the orbital order. Then we study the case of doping the perovskite B-site (Fe substitution for Mn) and show that the orbital ordering in Bi0.5Ca0.5Mn1-xFexO3 is suppressed at a critical dopant concentration. The doped compounds display an intricate phase diagram with ferromagnetic and antiferromagnetic states and transitions from magnetic semiconductors to charge ordered insulators. The structural results are correlated with data on charge carriers transport and magnetic properties.

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