Features of the intermediate state appearing between the C- and A-type orbital-ordered states in the highly-correlated e
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Features of the intermediate state appearing between the C- and A-type orbital-ordered states in the highly-correlated electronic system Sr1-xNdxMnO3 Ayumi Shiratani,1 Hiroki Sato,1 Yasuhide Inoue,2 and Yasumasa Koyama1,2 1 Department of Electronic and Physical Systems, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan 2 Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26, Nishiwaseda, Shinjuku-ku, Tokyo, 169-0051, Japan ABSTRACT The presence of the C- and A-type orbital-ordered states has been reported in the highlycorrelated electronic system Sr1-xNdxMnO3 (SNMO). The interesting feature of the oxide system is that an increase in the Nd content leads to the (C-type Ѝ A-type) state change across a temperature-independent morphotropic phase boundary (MPB). Although structural fluctuations can be expected near the MPB, the detailed features of the state change have not been understood sufficiently. Thus, the crystallographic features of the state change in SNMO with 0.35 ≤ x ≤ 0.49 have been investigated mainly at 300 K, by x-ray powder diffraction and transmission electron microscopy. It was found that the C-type orbital-ordered state with the tetragonalI4/mcm symmetry and the disordered orthorhombic-Imma state were present for 0.35 ≤ x ≤ 0.43 and for 0.45 ≤ x ≤ 0.49 at 300 K, respectively. The notable feature of the state change is that disordered regions with the cubic-Pm͵തm symmetry were also found locally for x = 0.43, in addition to the C-type state. Because the rotational-displacement pattern for oxygen octahedra involved in the disordered-Imma state is the same as that in the A-type state, furthermore, the former disordered state may be regarded as a precursor state to the A-type orbital ordering. INTRODUCTION There exist the C-type orbital-ordered (COO) and A-type (AOO) states in the highlycorrelated electronic system Sr1-xNdxMnO3 (SNMO). When Sr2+ ions in the end oxide SrMnO3 with the simple perovskite structure are replaced by Nd3+ ions, the COO state with the tetragonal-I4/mcm symmetry has been reported to change into the AOO state with the monoclinic-P21/m symmetry around x = 0.40 [1- 3]. According to the previous studies on the orbital-ordered states in SNMO, the state boundary between the COO and AOO states can be identified as a so-called morphotropic phase boundary (MPB), which is nearly parallel to the temperature axis in the state diagram. The feature of the crystal structures of these two orbitalordered states is that they commonly involve both the Jahn-Teller distortions to orbital orderings for eg electrons and the R25-type rotational displacement of oxygen octahedra [4, 5]. As for the difference in the rotational-displacement patterns, the COO state is characterized by the R25-type displacement only about one of the c axes in the pseudo-cubic notation, while the R25-type displacements about two axes are involved in the AOO state. Based on the difference, the (COO → AOO) state change across the temperature-independent MPB may take place i
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