Formation of the C-type Orbital-Ordered State in the Simple Perovskite Manganite Sr 1-x Sm x MnO 3
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Formation of the C-type Orbital-Ordered State in the Simple Perovskite Manganite Sr1-xSmxMnO3 Misato Yamagata1, Ayumi Shiratani1, Yasuhide Inoue2, and Yasumasa Koyama1,2 1 Department of Electronic and Physical System, Waseda University, Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan 2 Kagami Memorial Laboratory for Material Science and Technology, Waseda University, Nishiwaseda, Shinjuku-ku, Tokyo, 169-0051, Japan ABSTRACT The simple perovskite manganite Sr1-xSmxMnO3 (SSMO) has been reported to have a highly-correlated electronic system for eg-electrons in a Mn ion. According to the previous studies, the C-type orbital-ordered (COO) state with the I4/mcm symmetry was found to be formed from the disordered-cubic (DC) state on cooling. The feature of the COO state is that its crystal structure involves both the Jahn-Teller distortion to orbital ordering and the R25-type rotational displacement of oxygen octahedra. Because of the involvement of both the distortion and the displacement, their competition should be expected in the formation of the COO state. However, the detailed features of the competition have not been understood yet. Thus, the crystallographic features of the COO state in SSMO have been examined by x-ray powder diffraction and transmission electron microscopy. It was found that, when the Sm content increased from x = 0 at room temperature, the DC state changed into the COO state with the tetragonal symmetry around x = 0.13. The notable feature of the COO state is that the state is characterized by a nanometer-scaled banded structure consisting of an alternating array of two tetragonal bands. One tetragonal band consisted of the COO state involving both the Jahn-Teller distortion and the R25-type rotational displacement. But, there was only the latter displacement in the other, the state of which could be identified as a disordered tetragonal (DT) state. Based on this, it is understood that the COO-state formation from the DC state should take place via the appearance of the DT state, which may involve fluctuations of the C-type orbital ordering. INTRODUCTION The simple perovskite manganite Sr1-xSmxMnO3 (SSMO) has been reported to have a three-dimensional highly-correlated electronic system [1-10]. When Sr2+ ions in the end oxide SrMnO3 with only Mn4+ ions are partially replaced by Sm3+ ions, eg electrons are doped into its electronic system via the appearance of Mn3+ ions. The eg-electron doping results in fascinating orbital-ordered states such as the C-type orbital-ordered (COO) one. According to the previous studies concerning the orbital-ordered states in SSMO, the COO state with the tetragonal_
I4/mcm symmetry was obtained by cooling from the disordered cubic-Pm3m (DC) state around x = 0.40, and its crystal structure involves both the Jahn-Teller distortion to the C-type orbital ordering and the R25-type rotational displacement of oxygen octahedra [2,7-10]. In spite of the involvement of both the distortion and the displacement, however, their competition including the role of the R25-type displacem
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