Disordered Jahn-Teller-polaron states in the simple perovskite manganite Ca_1−xLa_xMnO₃ with 0.15 ≦ x ≦ 0.28

Among orbital-ordered states in manganites with strongly correlated electronic systems, both the C-type orbital-ordered (Ctype-OO) state and the charge- and orbital-ordered (COO) state have been reported in the simple perovskite Ca₁−_xLa_xMnO₃ (CLMO) with 0.15 ≦ x ≦ 0.28. To understand their formation from the disordered Pnma (DO) state with orthorhombic symmetry, the crystallographic features of prepared CLMO samples with 0.15 ≦ x ≦ 0.28 were examined mainly by transmission electron microscopy. When the temperature was lowered from the orthorhombic DO state for 0.15 ≦ x ≦ 0.23, referred to as Region I, the Ctype-OO state with monoclinic-P2₁=m symmetry appeared via a disordered Jahn-Teller polaron (DJTP) state, which exhibited characteristic diffuse scattering around each DO reflection in electron diffraction patterns. For 0.23 ≦ x ≦ 0.28 as Region II, cooling from the DO state resulted in the appearance of the COO state via a similar DJTP state. In patterns obtained from the COO state, satellite reflections characterizing the state were confirmed to be due to both transverse and longitudinal lattice modulations. It was also found that the DJTP states in Regions I and II were present in T_N ≧ T ≧ T_Ctype-OO and T ≧ T_COO, respectively, and were identified as the precursor states, which appear prior to the formation of the Ctype-OO and COO states.