Proton dynamics at low temperatures in Tl3H(SO4)2: possible quantum mechanical process of proton

Proton dynamics in the Tl3H(SO4)2 crystal has been examined from the measurement of the 1H spin–lattice relaxation time T1 below 120 K directly. It was found that T1 shows the steep increase at around 61 K with increasing temperature and the minimum at 15 K. From the behavior of T1 at around 15 K, the energy which is necessary for protons to move between two equilibrium positions in the hydrogen bond has been estimated to be 2.0 meV at low temperatures. This value is very low in comparison with the classical potential barrier height. Moreover, no dielectric anomalies are observed at around 15 K. These results indicate that quantum mechanical process of proton in the energy difference of 2.0 meV would exist at low temperatures in this crystal. In addition, the measured temperature dependence of T1 deviates from the calculated one above 61 K. It was also found from dielectric measurement that the anomaly of dielectric constant is observed at around 61 K. This dielectric behavior is different from that (antiferroelectric-like behavior) observed in the same type M3D1−xHx(XO4)2 (M=K, Rb; X=S, Se; x=0–1) crystals at low temperatures. These results indicate that the anomaly observed in the measurement of T1 and dielectric constant at around 61 K could be caused by a new type of phase transition or a new type of dielectric anomaly in the M3H(XO4)2 crystals accompanied by the change of proton dynamics.