Study of the ferroelectric–paraelectric phase transition of NaH3(SeO3)2 single crystals by 1H and 23Na NMR

The 1H and 23Na spin–lattice and spin–spin relaxation times of NaH3(SeO3)2 single crystals grown using the slow-evaporation method were measured as functions of temperature and frequency in the ferroelectric and paraelectric phases. The changes in the symmetry of the (SeO3)2− dimers as a result of the ferroelectric–paraelectric phase transition are associated with large changes in the spin–lattice and spin–spin relaxation times, and in the number of resonance lines. The large changes in the relaxation times at 195 K indicate that the H and Na ions are significantly affected by this transition. The change in the number of resonance lines for the 1H and 23Na nuclei means that the orientations of the (SeO3)2− dimers and the environments of the Na ions change at TC. Therefore, the orientations of the (SeO3)2− dimers and the environments of the Na ions play important roles in the phase transitions. In conclusion, the ferroelectric–paraelectric phase transition of NaH3(SeO3)2 is accompanied by changes in hydrogen-bond structure and distortions of the (SeO3)2− and Na+ ion lattices, which form a slightly distorted octahedron.