One-dimensional self-organization of water molecules in proton conducting Andersson-Wadsley titanates

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Layered alkali titanates with M 2 Ti 2 O 5 chemical formula (MTO, M=K,Rb) belonging to the Andersson-Wadsley perovskite family spontaneously incorporate water to form MTO.(H 2 O) x compounds, which exhibit superionic conductivity. At very low hydration x, scanning electron microscopy evidences one-dimensional heterogeneous patterns oriented along b that are arranged in an orderly manner. At higher hydration, the material is observed to spontaneously exfoliate by creating (001) surfaces. Simulations carried out using Density Functional Theory reveal an ordered arrangement of the guest water molecules in Rb 2 Ti 2 O 5, with strong hydrogen bonds between the water molecules and the apical oxygen of the host crystal. At low hydration x, the water molecules form self-organized one dimensional (1D) double chains along b. Further increase of the water content leads to the creation of hydrated (001)-surfaces that are made of densely packed water chains in agreement with the infrared spectroscopy measurements. Rb 2 Ti 2 O 5 exhibits highly anisotropic proton conductivity, with respect to the crystal orientation, with super-ionic conductivity along b reaching 3 mS/cm at room temperature after hydration. The combined observations and simulations suggest that these water chains are thus at the root of fast proton conduction, which is likely powered by a Grotthuss-like mechanism.