Fig. 1

We concentrate on the study of novel phenomena concerned with the water and proton in hydrogen-bonded systems. There are wide variety of chemical and biological materials constructed with hydrogen bonds in our sample storages, which are just regarded as a gift box (Fig. 1). For instance, various types of nano-porous crystals and superprotonic conductors like M₃H(XO₄)₂, M = K⁺, Rb⁺, Cs⁺; X = S, Se) are packed in the box. Moreover, we deal with the filmy samples of collagen (protein) and chitin (polysaccharide) that are fundamental elements of bone, skin, scales, and crab shells. We would like to study the physical properties of the hydrated and protonic states in those materials, and to create new functionalities.

Confined water in nanospaces and biological substances is an interdisciplinary subject in connection with physics, chemistry, biochemistry, medicine, mineralogy, geology, environmental and energy problems. We examine the proton conduction of molecular porous crystals with a hydrophilic nanochannel, in which the water lattice such as water nanotube or water chain is embedded. The shape and dimension of the nanochannel framework are chemically modified, and then various types of water lattices are easily obtained. We investigate the mechanisms of proton conduction through the water lattice and dipole ordering of water molecules, and the functionalities of gas sorption and nanofluidic channel.

We recently focus on the water nanotube consisting of tetradecahedral cages similar to methane hydrates. The water nanotube is stabilized even at room temperature and atmospheric pressure in contrast to methane hydrates that naturally exist under deep sea. Technological innovation is desired to remove methane molecules from the hydrate, and then CO₂ (greenhouse effect gas) is trapped in the cages. To establish such technology, we explore the sorption and diffusion of methane in the water nanotube. In addition, we examine how Xe is embedded in the water nanotube, because a Xe hydrate is formed at low pressure compared to a methane one. For a human body, Xe has an effect of general anesthesia, but the mechanism is not clear. About half a century ago, Linus Pauling proposed that a microcrystal of Xe hydrate inhibits the neural transmission, though his model has not been inspected experimentally. We successfully demonstrate that the proton conduction is huge suppressed by forming Xe hydrates in the water nanotube. This result opens the door to elucidate the mechanism of anesthesia in the human body.

For the sake of global warming prevention, electricity is hopefully generated by using renewable energy such as solar, wind and geothermal powers. Power-storage devices like Li-ion battery are important to reduce carbon dioxide emission, and additionally fuel cell is a powerful skill for clean electric generation, because the exhausted gas is water. A fuel-cell electrolyte without water molecules is desired for the next generation. A performance of fuel cell is dominated by the proton conduction via the lattice system. From solid-state physics point of view, a proton-lattice interaction is expected to play significant role in the conduction, while its knowledge is little obtained so far. Employing the typical superprotonic conductor M₃H(SeO₄)₂, we study the wide-range of electrodynamics to clarify the proton-lattice interaction, and the mechanisms of quantum-mechanical transfer and dielectric transition. The proton transfer in the dimeric selenates is found to hold several mechanisms due to hopping and phonon-assisted proton tunneling. Our recent task is to clear the cationic dependence (M⁺) of superprotonic and dielectric transition temperatures, and then we could obtain a design guideline for practical electrolytes.

To carry out the researches mentioned above, we measure complex conductivity and dielectric constant reflecting an electrodynamic response, and absorbance spectra relevant to molecular vibrations and optical transitions. As illustrated in Fig. 2, our experiments with electromagnetic waves cover in frequency ranges of radio wave, microwave, terahertz wave and infrared light. In case of a tiny sample, we often utilize a microscopic FT-IR spectrometer facilitated in SPring-8. Those apparatuses are combined with various experimental techniques according to temperatures, pressures, relative humidity, and samples. Furthermore, ab-initio, DFT and anharmonic-coupling calculations are helpful for the analysis and discussion of experimental results. From the detailed analysis of phonons and molecular vibrations, we intend to elucidate the physical property of proton-lattice interactions.

Fig. 2

To make aggressive use of our experimental skills and apparatuses, we collaborate many researchers in other institutions. Our group aims to build an interdisciplinary field beyond a framework of traditional solid-state physics. Our research and education is to train young talent, who is active in various fields of society in the future.