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

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Abstract
Deuteron and 23 Na magnetic resonances are measured of single crystals of NaD 3 (SeO 3 ) 2 between 134°K and 298°K to reveal the nature of the ferroelectric transition ( T c ∼267°K). Deuteron resonance lines D a and D b (relative intensity 1:2) above T c split into two and into four, respectively. The 23 Na resonance line above T c also splits into two below T c . Following conclusions are derived from measurements of temperature and angular dependences of the quadrupole splittings for these deuteron and 23 Na 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 T c . The phase transition is accompanied not only by order-disordering of hydrogen atoms in hydrogen bonds, but also by slight rotations of SeO 3 - ions and by a slight distortion of lattice in the neighborhood of Na + ions. The crystalline symmetry below T c is monoclinic, and the cell-doubling occurs along the b axis. Hydrogen-bond network below T c is estimated from the present study. This structure has permanent polarization in the ac plane.