Abstract
Structural distortions arising from the condensations of two essential kinds of phonon modes: the triply degenerate rotational modes ( ϕ x , ϕ y, ϕ z ) of MnO 6 and the doubly degenerate Jahn–Teller active modes ( Q 1, Q 2 ) have been systematically investigated in the perovskite manganites. Microstructural features associated with certain types of distortions have been observed by transmission electron microscopy (TEM). In RMnO 3 and La(Sr) 2NiO 4, we characterize the local structure, charge ordered states and orbital ordering by means of low-temperature TEM. We present direct evidence that the stripe modulation in La(Sr) 2NiO 4 is indeed one-dimensional within each NiO 2 plane. Several typical kinds of defect structures, including antiphase boundaries and the 90°-twin domains, appear commonly in the charge-ordered states.
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