Irreversible phase transition and spontaneous strain in CsHSO4

We have carried out DTA, X-ray measurements and the optical observation of domain structure using the polarizing microscope, in order to investigate the mechanism of irreversible phase transitions observed at around 350 K (TIII-IV) and 380 K (TII-III) in CsHSO4. It is found from the result of DTA measurement that with decreasing temperature from 430 K (in the phase I) the irreversible phase transitions at TIII-IV and TII-III are not observed at first day, but these irreversible phase transitions are observed after 12 days. Moreover, with increasing temperature, ferroelastic domain structure changes at III-IV and II-III phase and disappears with the superprotonic phase transition at 414 K (TI-II). It is also found that a lot of small ferroelastic domains are observed, after the temperature decreases from 430 K of phase I to 300 K. The size of these domains gradually increases, and no change of domain size is observed. Considering that the observed domains are ferroelastic ones, the change of domain structure is caused by the change of spontaneous strain. From these results it is deduced that the increase in spontaneous strain is closely related to the appearance of irreversible phase transition.