Stability of the orbital-modulated state in the layered perovskite manganite Sr_(2-x)RxMnO_4 (R = Pr, Nd)

摘要

To understand a stability of the orbital-modulated (OM) state in Sr_(2-x)R_xMnO_4 (R = Pr, Nd), the crystallographic features of oxide samples with 0.20 < x ≤ 0.50, prepared by the coprecipitation technique, have been investigated by transmission electron microscopy. When the Pr content x corresponding to the e_g-electron concentration increased from x = 0, the OM state appeared in 0.20 < x ≤ 0.40 as a neighboring state of an orbital ordered (OO) state around x = 0.20. The interesting features are that around x = 0.44 there is a nanometer-sized coexisting state consisting of the OM and disordered tetragonal (DT) states, and that an average size of OM-state regions in the coexisting state decreases with increasing the e_g-electron concentration. In addition, the so-called CE-type antiferromagnetic state found in Sr_(1.5)La_(0.5)MnO_4 is absent in the Nd and Pr cases. This suggests that the stability of the OM state is not directly associated with charge ordering of e_g electrons, but basically due to only their orbital ordering.