Characteristic features of the C-type orbital-ordered state in the simple perovskite manganite Sr_1-xNd_xMnO₃

In the highly-correlated electronic system Sr_1-x Nd_xMnO₃, the C-type orbital-ordered (COO) state is present for 0.15<x<0.38, and its crystal structure with the tetragonal-I4/mcm symmetry involves both the R₂₅-type rotational displacement of MnO₆ octahedra and the Jahn-Teller distortion as a response of a lattice system to orbital ordering. To understand the details of the competition between the rotational displacement and the Jahn-Teller distortion, the formation of the COO state from the disordered cubic (DC) state with the space group Pm3m has been investigated mainly by transmission electron microscopy. It was found that, when the temperature was lowered from the DC state for x = 0.20, for instance, COO regions with the tetragonal symmetry, exhibiting a {110}_DC banded structure, were locally formed in the matrix below about 330 K. The subsequent aging at 300 K resulted in the growth of COO regions; that is, the time-relaxation phenomenon. Because of the presence of antiphase boundaries for the rotational displacement in the matrix, the crystal structure of the matrix should also have the tetragonal symmetry. In other words, it is understood that the coexistence state appearing just after cooling from the DC state consists of two tetragonal regions with different c/a values. The coexistence state is apparently characteristic of the competition between the rotational displacement and the Jahn-Teller distortion for the formation of the COO state.