We have investigated the origin of the change in proton activity in the phase transition at TII-III(=369 K) in Cs3H(SeO4)2from the viewpoint of its ferroelasticity by using1H NMR and X-ray measurements. It is found that the second moment of the1H NMR absorption line rapidly decreases at TII-IIIwith increasing temperature. From this result, we conclude that the hopping motion of a proton, which is the precursor motion in the superprotonic phase, becomes more active above TII-III. This result is consistent with the fact that the electrical conductivity in phase II is larger than that in phase III. Furthermore, it is also found that the spontaneous strain decreases abruptly at TII-III. From these results, it is deduced that the decrease in the spontaneous strain at TII-IIIcauses the increase in the proton activity at TII-III. In addition, it is deduced that the increase in proton activity and the decrease in the spontaneous strain at TII-IIIare closely related with the appearance of the superprotonic phase transition at TI-II(=456 K). © 2005 Elsevier Ltd. All rights reserved.
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