Crystallographic features of orbital ordering related to the C-type antiferromagnetic state in the simple perovskite man
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Crystallographic features of orbital ordering related to the C-type antiferromagnetic state in the simple perovskite manganite Ca1-xPrxMnO3 Kentaro Kojima,1 Yasuhide Inoue,2 and Yasumasa Koyama1,2 1 Department of Electronic and Physical system, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan 2 Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-826, Nishiwaseda, Shinjuku-ku, Tokyo, 169-0051, Japan ABSTRACT In the highly-correlated electronic system Ca1-xPrxMnO3 having the simple perovskite structure, it has been reported that there exists the C-type orbital-ordered (COO) state accompanying an antiferromagnetic ordering for 0.10 ≤ x ≤ 0.25. According to the previous studies concerning orbital-ordered states in simple perovskite manganites, the COO state was understood to be characterized by a spatial array of (3z2-r2)-type orbitals for 3d electrons in Mn ions. The notable feature of the COO state in Ca1-xPrxMnO3 is that the state with the monoclinicP21/m symmetry appears as a result of the structural transition from the disordered state with the _
orthorhombic-Pnma symmetry. Compared with the COO-state formation from the cubic-Pm3m state, however, the formation from the disordered-Pnma state has not been understood yet. We have thus examined the crystallographic features of the formation of the COO state in Ca1xPrxMnO3, mainly by x-ray powder diffraction and transmission electron microscopy. In the case of x = 0.16, for instance, the COO state was found to appear from the disordered-Pnma state around 90 K on cooling. The notable feature of the formation is that, in the Pnma state just before the COO-state formation, characteristic diffuse scattering appeared around each reflection in electron diffraction patterns, together with the splitting of the 200c reflection in x-ray powder diffraction profiles in the pseudo-cubic notation. Based on these experimental data, it is understood that the formation of the COO state in Ca1-xPrxMnO3 accompanies remarkable fluctuations of the C-type orbital ordering in the disordered-Pnma state. INTRODUCTION The simple perovskite manganites with the highly-correlated electronic system have been reported to exhibit interesting electronic states that are associated with degrees of charge, orbital, and spin freedoms for 3d electrons in Mn ions [1-5]. Among these electronic states, orbitalordered state such as the C-type orbital-ordered (COO) state accompanying an antiferromagnetic ordering have been found in simple-perovskite systems such as Sr1-xRxMnO3 and Ca1-xRxMnO3 (R: rare-earth ions). The notable feature of the COO-state formation is that the state in Sr1_
xRxMnO3
is, for instance, formed from the disordered state with the cubic-Pm3m symmetry, in which eg electrons for orbital ordering are well defined. On the other hand, the disordered state in Ca1-xRxMnO3 has the orthorhombic-Pnma symmetry, which is apparently lower than the cubic one. In the case of Ca1-xPrxMnO3 (CPMO) with R = Pr, in particular, the COO state with the mono
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