Buckled Layers in K0.66Mn2O4·0.28H2O and K0.99Mn3O6·1.25H2O Synthesized at High Pressure: Implication for the Mechanism of Layer-to-Tunnel Transformation in Manganese Oxides

A 2 × 2 layer-buckled manganese oxide, K0.66Mn2O4·0.28H2O (I), has been synthesized under high pressure and retained at ambient pressure; it is metastable and will finally transform to a 2 × 1 layer-buckled K0.99Mn3O6·1.25H2O (II) in 1 year. Both crystal structures were determined by single-crystal X-ray diffraction. On the basis of these buckled layers, which are a result of ordering of Mn3+/Mn4+ in separate rows and cooperative Jahn−Teller distortion of Mn3+O6 octahedra, a mechanism of structure transformation from birnessite to tunnel structures was proposed.