Commensurate-incommensurate phase transition in charge ordered La 1-x Ca x MnO 3 by single crystal diffraction and elect
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Commensurate-incommensurate phase transition in charge ordered La1-xCaxMnO3 by single crystal diffraction and electron imaging Jing Tao, Y Murakami1 and Jian-min Zuo Department of Material Science and Engineering and Materials Research Laboratory University of Illinois at Urbana-Champaign 1304 West Green Street Urbana, Illinois 61801, U.S.A 1 Institute of Multidisciplinary Research for Advanced Materials Tohoku University Sendai, 980-8577, Japan ABSTRACT Electron nanodiffraction from single crystal domain of La1-xCaxMnO3 with 0.5 ≤ x ≤ 0.8 is carried out to study the charge ordering phase transition in this system. The melting processes of charge ordering structure in samples with x = 1/2, 2/3 and 4/5 are observed in-situ by increasing temperature from 93K – 298K. During the phase transition, electron diffraction results show that the nearly commensurate low-temperature charge ordering structure continuously evolves to incommensurate structure with directly decreasing sizes of charge ordered area. The picture of this phase transition is confirmed by the real space imaging. The real space images clearly show the introduction of charge ordering defects during the melting process in this sample. INTRODUCTION Charge ordering, and phase separation in general, has been shown as the key to understand colossal magnetoresistance and other properties of complex oxides [1, 2]. The ratio of Mn3+ and Mn4+ varies with the doping concentration of Ca in La1-xCaxMnO3. At low temperature, the symmetry of the Mn3+ octahedron is lowered by one electron in eg orbital (Jahn-Teller distortion) while the Mn4+ octahedron has the same symmetry [3]. When the Ca concentration is from 0.5 to 0.8 in the phase diagram [4], the compound undergoes a charge ordering phase transition [5] accompanied by a metal-insulator phase transition with decreasing temperature. At room temperature, the structure is a highly distorted perovskite. The low-temperature charge ordering and the Jahn-Teller effect induce further small atomic displacements on top of the distorted perovskite structure. The charge ordering structure is nearly commensurate at x = 1/2, 2/3, 3/4…. The ordered atomic displacement at x = 1/2 and 2/3 can be described within the framework of Wigner-crystal model, supported by x-ray powder diffraction [6] and electron diffraction [7]. This paper focuses on the atomic displacement associated with charge ordering and how it evolves with temperature. We monitored the evolution of atomic displacement by quantitative electron diffraction measurement of the superlattice peaks and by direct electron imaging during the charge ordering phase transition. The results show the importance of defect formation during the phase transition.
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EXPERIMENT DETAILS The polycrystalline bulk samples of La1-xCaxMnO3 (x = 1/2, 2/3, 4/5) were synthesized using standard ceramics method described elsewhere [1]. Power x-ray diffraction data confirm the single-phase of each sample. The quality of sample has been characterized by the magnetization and transpor
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