Phase transitions in trithallium hydrogen disulfate Tl3H(SO4)2

Differential scanning calorimetry (DSC), measurements of the complex dielectric constant and NMR measurements were carried out for Tl3H(SO4)2 below room temperature. It was found from the measurements of DSC and the complex dielectric constants that Tl3H(SO4)2 undergoes phase transitions at TI–II=267 K, TII–III=239 K, TIII–IV=196 K and TIV–V=156 K on heating. It was also found that electric conductivity along the hexagonal c axis is about 2×10−5 S/cm at room temperature. The activation energies of the phases I and II are determined to be 0.42 and 0.46 eV, respectively. These values are close to the activation energies in the superionic phase of other M3H(XO4)2 type compounds. Moreover, the width of the proton NMR absorption line in the phases I and II becomes considerably narrower than that at liquid nitrogen temperature due to motional narrowing of the absorption line. These facts indicate that this crystal displays superionic conduction in the phases I and II. In addition, the transition entropies for the observed phase transitions were estimated from the DSC measurement. The largest transition entropy is observed at 239 K. This value (8.40 J/mol K) is close to R ln 3. This result indicates that the phase transition at 239 K is the superionic transition.