Protonic Diffusion at High Pressure

The experimental fact of the small diffusion rate at ambient pressure and the theoretical prediction of a superprotonic state under extreme conditions point us to diffusion measurements for hot ice in which protons are thermally activated to move faster. The high pressure techniques enable us to generate an extreme condition around 1000 K and 20 GPa where ice is predicted to enter the superprotonic state. As seen in Fig. 24.1, ice VII exists over a wide pressure span above 2 GPa. The interpenetrated dense structure would prevent water molecules from moving and allow measurement of the protonic diffusion at high temperatures beyond the melting point of 273 K at ambient pressure [18]. Ice VII thus provides a great advantage for protonic diffusion measurement. 

A diamond-anvil-cell (DAG) is a small high pressure cell most suitable for the spectroscopic measurement of molecular or atomic diffusion. The DAG is used for various kinds of spectroscopic investigations on liquids and solids at pressures up to several tens of GPa [19-22]. The optically transparent nature of diamond over a wide wavelength span allows in situ optical measurements in combination with conventional equipment such as visible light or infrared spectrometers. The protonic diffusion in ice is measured by a traditional diffusion-couple method, in the present case, with an H2O/D2O ice bilayer. The mutual diffusion of hydrogen (H) and deuteron (D) in the ice bUayer is monitored by measuring the infrared vibrational spectra. The experimental details are described in the following sections. 

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