Superprotonic and ferroelastic phase transition in K3H(SO4)2

Measurements of differential scanning calorimetry, complex dielectric constant, proton NMR and optical observations have been carried out for K3H(SO4)2 above room temperature. It was found that K3H(SO4)2 undergoes a phase transition at Tc=478 K below the decomposition temperature of 535 K. The transition entropy is estimated to be 8.37 J/mol K, close to that at the superprotonic phase transition of M3H(SeO4)2 crystals (M=K, Rb and Cs).Moreover, we have observed the step-wise increases of the electrical conductivity with the increase of temperature and good conductivity above Tc. Furthermore, above Tc, we have observed the motional narrowing of the 1H NMR line caused by the mobile proton. In addition, we found from the optical observations that K3H(SO4)2 undergoes the ferroelastic phase transition at Tc. These results suggest that K3H(SO4)2 exhibits the superprotonic conductivity above Tc and the ferroelasticity below Tc as in M3H(SeO4)2 crystals.