Possible orbital-ordered state in the highly-correlated electronic material Sr_1-xCe_xMnO₃

The simple perovskite manganite Sr_1-xCe_xMnO₃ (SCMO) has a highly-correlated electronic system with a three-dimensional character. Because the presence of orbital-ordered states of eg electrons can be expected in SCMO, the crystallographic features of SCMO samples with 0.09 ≤ x ≤ 0.20 have been investigated mainly by transmission electron microscopy. In addition to fundamental reflections due to the simple perovskite structure, their electron diffraction patterns at room temperature exhibited both the presence of superlattice reflections at in the cubic notation and the splitting of fundamental and superlattice reflections. The careful analysis of these reflections indicated that the superlattice reflections originated from the R₂₅-type rotational displacement of oxygen octahedra about one of the <100>_c directions. On the other hand, the splitting of the reflections was found to be due to a {110}_c banded structure consisting of two tetragonal bands with different c/a values. Because one of two tetragonal bands had the c/a value of about 1.028, the splitting reflects the introduction of the Jahn-Teller distortion as a response of a lattice system to orbital ordering. It is thus understood that the C-type orbital ordering of e_g electrons should be involved in the state at room temperature for 0.09 ≤ x ≤ 0.20 in SCMO.