Phase transitions of the fast-ion conductorK3H(SeO4)2 studiedby 1H and 39K NMR spectroscopy

Abstract

The 1H and39K spin–latticerelaxation, T1, spin–spinrelaxation time, T2 in the laboratory frame, and the spin–lattice relaxation time in the rotating frameT1ρ insuperionic K3H(SeO4)2 single crystals grown by the slow evaporation method were measured in the ferroelastic and paraelasticphases. The 1H T1, T1ρ,and T2 exhibited different trends with temperature in the ferroelastic phase, but werefound to have very similar, liquid-like values in the paraelastic phase. In theparaelastic phase, the reorientational and translational motions are lost, andthe proton motion can be characterized with a single correlation frequency,ωc. The observationthat the variations of 1H T1, T1ρ,and T2 of K3H(SeO4)2 crystals with temperature are very similar in the paraelastic phase indicates that the destructionand reconstruction of hydrogen bonds does indeed occur at high temperatures. In addition, the39K T1 and T2 were found to be similar at high temperatures, as was also observed for the1H T1 and T2. The crystal in the high-temperature phase is a fast ionic conductor. The motiongiving rise to this liquid-like behaviour may be related to superionic motion.