Abstract
In order to investigate why crystal symmetry lowers with increasing temperature by phase transition of TII–III (=369 K) in Cs3H(SeO4)2, in spite of the fact that crystal symmetry in the high-temperature phase of many ionic conductors becomes higher by the phase transition, we have studied the relation between the change in crystal symmetry and the appearance of proton motion. It was found from the analysis of domains based on crystal structure that the number of possible geometrical arrangement of hydrogen bond in phase II becomes two times larger than that in phase III, derived from the lowering of crystal symmetry with increasing temperature. These results indicate that the lowering of crystal symmetry in phase II appears by the increase of the number of geometrical arrangements and by the enhancement of the flexibility of hydrogen bond. Considering that the enhancement of the flexibility of hydrogen bond yields mobile proton in phase II, it is deduced that mobile proton in phase II appears in exchange for the lowering of crystal symmetry at II–III phase transition.
Highlights
It is known that M3H(XO4)2 compounds (M = K, Rb, Cs; X = S, Se) exhibit a superprotonic conductivity and becomes the electrolyte of the fuel cell [1,2,3,4,5,6,7,8,9,10,11]
In order to investigate the origin of the lowering of crystal symmetry, we investigated the change in possible hydrogen bond patterns by phase transition of TII–III on the basis of the crystal and domain structures
From the analyses of the hydrogen-bond network around TII–III, we found that the number of the network patterns of hydrogen bonds in phase II is two times larger than that in phase III
Summary
It is known that M3H(XO4) compounds (M = K, Rb, Cs; X = S, Se) exhibit a superprotonic conductivity and becomes the electrolyte of the fuel cell [1,2,3,4,5,6,7,8,9,10,11]. The Cs3H(SeO4) crystal, which is one of M3H(XO4) superprotonic conductors, exhibits a superprotonic conductivity above 456 K (=TI–II). Cs3H(SeO4) shows the interesting feature that the phase transition from the low-temperature monoclinic-C2/m phase (phase III) to high-temperature monoclinic-A2/a phase (phase II) exists at 369 K (=TII–III) [14,15,16,17,18]. In conjunction with the phase transition at TII–III, some interesting phenomena are observed [18]. For example: (1) Mobile proton appears above TII–III, as shown in the motional narrowing of NMR line width in Figure 1a; and (2) the increase of the electrical conductivity is observed above TII–III as a precursor effect of superprotonic conductivity (Figure 1b) [18]
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