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

本研究的目的是阐明具有代表性的固体-酸型质子导体——硫酸氢铯（CsHSO_4）的质子扩散系数与各晶相结构之间的关系。在温度-压力相图中发现CsHSO_4含有许多多晶。多晶体都具有相同的化学成分，但每种晶体结构的质子电导率不同。首先，利用红外光谱和拉曼光谱检测了CsHSO_4晶体的氢键态，并绘制了温度-压力相图。然后发现新的相HPHT1和HPHT2。这些相在室温高压下是稳定的。利用红外测图方法，成功地跟踪了高压下样品中O-D分布的时间变化。我们测量了CsHSO_4/CsHSO_4固体中质子-氘核相互扩散系数在373 K时的压强依赖性。第二阶段的系数几乎不变，直至1gpa。我们的结果表明，压缩对离子重定向运动的影响很小，尽管离子应该紧密聚集在一起。在1.5 GPa时，从II相到HPHT1相的扩散系数下降到1×10^<-16> m^2/s，在HPHT2相中也有类似的数值。从II相到HPHT1的结构变化会导致相互扩散系数降低。结果表明，HPHT1和HPHT2相均抑制了HSO_4^￣离子的重取向运动。在银沉积板上获得了CsHSO_4的表面增强红外吸收光谱。但是，应用扩散系数测量还存在一些问题（如试样变薄、板的耐酸性等）。

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.