The synthesis of a novel birnessite structure manganese oxide, Cs0.24MnO2, via a modified sol-gel route is reported in this work. The product was characterized by X-ray diffraction （XRD）, transmission electron microscopy （TEM）, thermogravimetric analysis （TGA）, and magnetic susceptibility. It is found that Cs0.24MnO2 crystallizes in a monoclinic phase with a nanosheet morphology. With lowering the temperature, Cs0.24MnO2 shows an antiferromagnetic transition at about 43.8 K, which is different from its paramagnetic K-counterpart. The effective moment of Mn ions in Cs0.24MnO2 is determined to be 4.2 μB, indicating a mixed valence of Mng＋/Mn3＋.
