Hydrogen-Bond Network and Ferroelectric Transition in Sodium Trihydrogen Selenite, NaD3(SeO3)2 by Nuclear Magnetic Resonance Study

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Deuteron and 23Na magnetic resonances are measured of single crystals of NaD3(SeO3)2 between 134°K and 298°K to reveal the nature of the ferroelectric transition (Tc∼267°K). Deuteron resonance lines Da and Db (relative intensity 1:2) above Tc split into two and into four, respectively. The 23Na resonance line above Tc also splits into two below Tc. Following conclusions are derived from measurements of temperature and angular dependences of the quadrupole splittings for these deuteron and 23Na resonance lines. All the hydrogen atoms are linked with moderately strong hydrogen bonds, and are jumping back and forth in the double well potential above Tc. The phase transition is accompanied not only by order-disordering of hydrogen atoms in hydrogen bonds, but also by slight rotations of SeO3−− ions and by a slight distortion of lattice in the neighborhood of Na+ ions. The crystalline symmetry below Tc is monoclinic, and the cell-doubling occurs along the b axis. Hydrogen-bond network below Tc is estimated from the present study. This structure has permanent polarization in the ac plane.
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