Crystallographic features of the orbital-ordered, and charge-and-orbital- ordered states in Sr2-x Ndx Mn O4

The crystallographic features of the orbital-ordered and charge-and-orbital-ordered states in Sr2-x Ndx Mn O4 have been investigated by in situ observations, using a transmission electron microscope, in an attempt to understand the distinct characteristics of their electronic states. In the orbital-ordered state with orthorhombic symmetry, there exist four banded-domain-structure states, that is, two orthorhombic variant states (OI + OII) accompanying the C -type antiferromagnetic ordering at lower temperatures and (DT+ OI) coexistence and (OI + OII) states lacking magnetic ordering at higher temperatures, where DT represents the disordered tetragonal state and OI and OII the two orthorhombic variants. On the other hand, the stability of the charge-and-orbital-ordered state present for 0.25≤x≤0.43 is strongly suppressed when x>0.38. As a result of the strong suppression, the charge-exchange-type charge-and-orbital-ordered state is absent for x=0.5. The important features of the charge-and-orbital-ordered state are that it is basically characterized by incommensurate structural modulations and that round-shaped domains separated by disordered regions appear for 0.38<x≤0.43 near the DT state. On the basis of these data, the origin of the appearance of the four banded-structure states in the orbital-ordered state was attributed to a coupling between the local Jahn-Teller distortion and the long-range distortions, including a dilational one. In addition, we propose a model for the ground-state change between the orbital-ordered and charge-and-orbital-ordered states in terms of orbital degree of freedom.