Abstract: A series of perovskite compounds of La_1-x-yCa_xK_yMnO₃ was prepared by hydrothermal disproportionation reaction of MnO₂. The three oxidation states of manganese(Mn³⁺, Mn⁴⁺ and Mn⁵⁺) can stably exist in the compounds. Under room temperature conditions, twice and three times modulated structure produced by the charge ordering can be observed by X-ray diffraction(XRD), select area electron diffraction(SAED) and high resolution transmission electron microscopy(HR-TEM). The splits of the diffraction peaks of the single crystals confirmed by synchrotron X-ray diffraction experiment are in accordance with those of the powder sample. The existence of the different oxidation states of Mn³⁺, Mn⁴⁺ and Mn⁵⁺ can be considered to be the dominating reason of the complicated room temperature modulated structure.

Key words: Modulated structure, Mixed valence state, Room temperature