Crystallographic features of the charge-orbital-ordered state in the highly-correlated electronic system Ca_1-xPr_xMnO₃

The charge-exchange-type orbital-ordered (CEOO) state accompanying antiferromagnetic ordering has been reported in the highly-correlated electronic system Ca_1-xPr_xMnO₃ (CPMO) with the simple perovskite structure. The feature of the CEOO state in CPMO is that the state is present in the wide Pr-content range of 0.30 ≤ x ≤ 0.70. Although the Zener-polaron model was proposed for the CEOO state at x = 0.60, the detailed features of the CEOO state, particularly for lower Pr contents, have not been understood sufficiently. We have thus investigated the crystallographic features of the CEOO state in CPMO with 0.40 ≤ x ≤ 0.50, mainly by transmission electron microscopy. It was found that, when the temperature was lowered from the disordered state, incommensurate satellite reflections characterizing the CEOO state appeared in electron diffraction patterns below about 250 K in CPMO. The careful analysis of the reflections indicated that the CEOO state for 0.40 ≤ x ≤ 0.45 was different from that for 0.45 ≤ x ≤ 0.50. Concretely, the former CEOO state accompanied only a transverse lattice modulation, while both transverse and longitudinal modulations with different wave vectors appeared in the latter state. This implies that the state for 0.40 ≤ x ≤ 0.45 can be regarded as an orbital-modulated (OM) state without a charge modulation. It is thus understood that an increase in the Pr content leads to the OM-to-CEOO state change below about 250 K around x = 0.45.