Infrared measurement on proton diffusion coefficients for crystalline phases of cesium hydrogen sulfate

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The purpose of this study was to clarify a correlation between the proton diffusion coefficients and the structures for various crystalline phases of cesium hydrogen sulfate (CsHSO_4), which is a representative solid-acid type proton conductor. CsHSO_4 have been shown to contain many polymorphs in the temperature-pressure phase diagram. The polymorphs all have the same chemical composition but show different proton conductivities for each crystal structure. At first, we examined the hydrogen bond states of CsHSO_4 crystal by infrared and Raman spectra, and determined a temperature-pressure phase diagram. Then, new phases HPHT1 and HPHT2 were found. These phases were stable at room temperature under high pressure. We succeeded in tracking the temporal change in the O-D distribution in the sample at high pressures using the infrared mapping measurement. We measured pressure dependence of a proton-deuteron mutual diffusion coefficient in a CsHSO_4/CsHSO_4 solid at 373 K. The coefficient for phase II was almost constant up to 1 GPa. Our results indicated that the influence of compression on the reorientation motion is small in phase II, although the ions should gather closely together. The diffusion coefficient decreased to 1×10^<-16> m^2/s at the transition from phase II to phase HPHT1 at 1.5 GPa, and showed similar values in phase HPHT2. The structural change from phase II to HPHT1 would cause a decrease in the mutual diffusion coefficient. The results showed that the reorientation motion of the HSO_4^￣ion was suppressed in both phases HPHT1 and HPHT2. Surface enhanced infrared absorption spectra of CsHSO_4 were obtained on a silver deposition plate. However, some problem (for example, thinning of tie sample, acid-resistance of plate) remains to apply the diffusion coefficient measurement.