Relaxation phenomenon in the formation of the C-type orbital-ordered state in the simple perovskite manganite Sr_1-xNd_xMnO₃

The formation of the C-type orbital-ordered (COO) state from the disordered cubic (DC) state in Sr_1-xNd_xMnO₃ (SNMO) with the simple perovskite structure has been examined mainly by transmission electron microscopy. As the COO state has tetragonal I4/mcm symmetry, its formation is associated with the cubic-to-tetragonal structural transition. It was found that, when SNMO samples were cooled down from the DC state, the R₂₅-type rotational displacement of MnO₆ octahedra was first induced, together with the symmetry change into the tetragonal I4/mcm structure. The C-type orbital ordering then appeared due to the induction of the Jahn-Teller distortion as a response of a lattice system to this orbital ordering. Because no symmetry change occurred in the latter case, the Jahn-Teller distortion can be regarded as a dilatational distortion. One interesting feature is that the appearance of the dilatational Jahn-Teller distortion led to a nanometer-scale banded structure, characterized by an alternating array of two tetragonal variants with different c/a values. In addition, the formation of the COO state from the DC state exhibited a time-relaxation phenomenon. The origin of this relaxation phenomenon is also discussed here in terms of the competition between the tetragonal spontaneous strain for the rotational displacement and the dilatational Jahn-Teller distortion for orbital ordering.