Investigation of the crystal symmetry of BiMnO₃: electron diffraction study

Abstract

state conversion. Ordering of the spin states is manifested in the corresponding superstructure reflections; these can be traced in a diffraction experiment as a function of external stimuli. By mapping reciprocal space with an area detector and synchrotron light, we have studied the temperature dependence of the superstructure reflections for NdBaCo2O5.5 and TbBaCo2O5.48. We have found that above the metal-insulator transition there are two different Co ions in the asymmetric unit, one sitting in a pyramidal and one in an octahedral environment. Below the transition temperature there are four structurally different Co ions. This observation agrees with the "spin blockade" mechanism suggested for the metal-insulator transition in cobaltites. We also present results of structural analyzes illustrating how the corresponding powder diffraction measurements could easily overlook the correct structure. A symmetry analysis bracketing the observed phase transitions within the context of Landau theory is also given.

Crystal and Magnetic Structures and Properties of BiMnO3+δ

“…Therefore, thin-film samples may be slightly oxygen deficient and have various symmetries. Observation of a long-range ordered structure with the C 2 symmetry and a short-range ordered structure with the P 2 or P 2 1 symmetry in BiMnO 3 by selected-area electron diffraction can also be explained by the transformation inside electron miscroscopes . However, we should note that oxygen-deficient BiMnO 3−δ samples cannot be prepared by direct high-pressure high-temperature synthesis, similar to oxygen-deficient LaMnO 3−δ samples .…”

“…Neutron powder diffraction data showed that the average structure of bulk BiMnO 3 is indeed very well described by the C 2/ c model. , Single-crystal structural studies of BiMnO 3 also confirmed the C 2/ c symmetry . In terms of convergent-beam electron diffraction, the space group of bulk BiMnO 3 was determined to be C 2/ c . However, careful selected-area electron diffraction studies showed a long-range ordered structure with C 2 symmetry and a short-range ordered structure with P 2 or P 2 1 symmetry .…”

“…In terms of convergent-beam electron diffraction, the space group of bulk BiMnO 3 was determined to be C 2/ c . However, careful selected-area electron diffraction studies showed a long-range ordered structure with C 2 symmetry and a short-range ordered structure with P 2 or P 2 1 symmetry . The local symmetry of bulk BiMnO 3 was suggested to be P 2 or P 2 1 by atomic pair distribution function analysis .…”

“…18 However, careful selected-area electron diffraction studies showed a long-range ordered structure with C2 symmetry and a short-range ordered structure with P2 or P2 1 symmetry. 18 The local symmetry of bulk BiMnO 3 was suggested to be P2 or P2 1 by atomic pair distribution function analysis. 19 It is possible that the oxygen content plays a crucial role.…”

Low-Temperature Vacuum Reduction of BiMnO3

“…17 With a stronger beam, additional weak spots on electron diffraction patterns have been observed corresponding to a longrange-ordered structure with the C2 symmetry and a short-rangeordered structure with the P2 or P2 1 symmetry. 32 The presence of the additional spots could be either intrinsic or just caused by an initial transformation inside an electron microscope. With longer exposure time, 27 the monoclinic-to-cubic transformation occurs.…”