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酸型质子导体硫酸铯(CsHSO4)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，CsHSO_4含有多种晶型。这些多晶型都具有相同的化学组成，但对于每种晶体结构显示不同的质子电导率。首先，我们用红外光谱和拉曼光谱研究了CsHSO4晶体的氢键态，并确定了温度-压力相图。然后，发现了新的相HPHT1和HPHT2。这些相在室温高压下是稳定的。我们利用红外测绘测量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373K下CsHSO4/CsHSO4固体中质子-氚互扩散系数与压力的关系。我们的结果表明，在第二相中，压缩对重定向运动的影响很小，尽管离子应该紧密地聚集在一起。在1.5 GPaII相向HPHT1相转变时，扩散系数降至1×10^-16；m^2/S，而HPHT2相的扩散系数与之相近。从第二相到HPHT1相的结构变化会导致互扩散系数的下降。结果表明，在HPHT1和HPHT2两相中，HSO4、￣离子的重取向运动均受到抑制。在镀银板上获得了CsHSO4的表面增强红外吸收光谱。然而，扩散系数的测量还存在一些问题(如样品变薄、板材耐酸性等)。

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.