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

Abstract

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 (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 ²³Na 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 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 T_c. 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 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.