Proton conductivity and spontaneous strain below superprotonic phase transition in Rb3H(SeO4)2

We have carried out the 1H-NMR and X-ray measurements, in order to reveal the origin of the relatively high electrical conductivity observed even below a superprotonic phase transition at Tc (= 449 K) in Rb3H(SeO4)2. It is found that the second moment of the 1H-NMR absorption line rapidly decreases around 338 K with increasing temperature. This result indicates that the hopping motion of a proton, which is the precursor motion of proton motion in the superprotonic phase, appears around 338 K. This result indicates that the increase in the proton mobility above 338 K is closely related with the increase of slope in the temperature dependence of the electrical conductivity around 338 K with increasing temperature. Furthermore a spontaneous strain obtained from the X-ray diffraction data steeply decreases around 338 K. From these results, it is deduced that the increase in the proton mobility above 338 K is caused by the same mechanism as the decrease in the spontaneous strain and yields the proton conductivity observed even below Tc.