In the highly-correlated electronic system Ca1-xPrxMnO3 having the simple perovskite structure, it has been
reported that there exists the C-type orbital-ordered (COO) state accompanying
an antiferromagnetic ordering for 0.10 ≤ x
≤ 0.25. According to the previous studies concerning orbital-ordered
states in simple perovskite manganites, the COO state was understood to be
characterized by a spatial array of (3z2-r2)-type orbitals
for 3d electrons in Mn ions. The notable feature of the COO state in
Ca1-xPrxMnO3 is that the state with the monoclinic-P21/m
symmetry appears as a result of the structural transition from the disordered
state with the orthorhombic-Pnma symmetry. Compared with the COO-state formation
from the cubic-Pm$\overline 3$m state, however, the formation from the disordered-Pnma state
has not been understood yet. We have thus examined the crystallographic features
of the formation of the COO state in Ca1-xPrxMnO3, mainly by x-ray powder diffraction and transmission
electron microscopy. In the case of x = 0.16, for
instance, the COO state was found to appear from the disordered-Pnma state
around 90 K on cooling. The notable feature of the formation is that, in the
Pnma state just before the COO-state formation, characteristic diffuse
scattering appeared around each reflection in electron diffraction patterns,
together with the splitting of the 200c reflection in x-ray powder diffraction
profiles in the pseudo-cubic notation. Based on these experimental data, it is
understood that the formation of the COO state in Ca1-xPrxMnO3 accompanies remarkable fluctuations of the C-type orbital
ordering in the disordered-Pnma state.