Two-Dimensionally confined water in between MnO2 layers of Na-Birnessite

Two-dimensionally confined water in between the MnO2 octahedral layers in Na-birnessite has been studied to clarify the physical properties and the structural arrangements. From the analysis employing an inductively coupled plasma (ICP) method and chemical titration, the chemical composition is determined to be Na _0.28Mn ⁴⁺ _0.73Mn ⁴⁺ _0.27O ₂ yH ₂O in our sample. According to the X-ray diffraction (XRD) experiments, the interlayer distance along the c axis is 7.17 A at 293 K in atmosphere, though the distance shrinks to 5.77 when the sample is kept either at 333 K in atmosphere or at 293 K in vacuum. The weight loss obtained by thermogravimetry exhibits two steps caused by the dehydration in atmosphere. Together with the XRD data, we confirm that there are two contributions of the water in the interlayers; weakly hydrated water and strongly hydrated water. The latter is regarded as a structural water to maintain the layer structure. In the infrared spectra, three novel peaks are found to develop in evacuating the sample. These peaks are associated with three different distances in between oxygen atoms of water molecules identified by the precise X-ray studies so far. We consider that Napion locates approximately at one MnO2 unit with Mn ³⁺ among the four MnO ₂ units to preserve charge valence, and the strongly hydrated water molecules surround Napin the form of a quasi-planer hexamer-like cluster reflecting the symmetry of MnO ₂ layer. The strict restriction of water to the plane and the Coulomb repulsion between Napmay induce a deformation of the cluster leading to the abovementioned three different distances.