Stability of the orbital-modulated state in the layeredperovskite manganite Sr_(2-x)R_xMnO_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 eg-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 eg-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 eg electrons, but basically due to only their orbital ordering.