Role of the tilting of oxygen octahedra in the stabilization of the orbital-modulated state in the layered perovskite manganite Ca_2-xNd_xMnO₄

There exists the orbital-modulated (OM) state in the layered manganite Ca_2-xNd_xMnO₄ (CNMO) with 0.20 ≤ x ≤ 0.50. To understand the effect of the tilting of MnO₆ octahedra to the stability of the OM state, the crystallographic features of CNMO samples prepared by a solid-state reaction have been investigated mainly by transmission electron microscopy. For 0.30 ≤ x ≤ 0.50, it was found that the (LTO → Pccn/LTT) structural transition occurred on cooling from room temperature, where the LTO and Pccn/LTT structures are, respectively, characterized by tilting displacements of oxygen octahedra about one and two of the <110> directions. The notable feature of the (LTO → Pccn/LTT) transition is that its progress strongly suppresses the growth of the OM state for 0.30 ≤ x ≤ 0.45. As a result of the suppression, the OM state exhibits a re-entrant behavior for its appearance. This is an indication that the Pccn/LTT tilting is not favorable for the stabilization of the OM state.