Nuclear-Magnetic-Resonance Study of the Ferroelectric Transitions in NaH3(SeO3)2 and NaD3(SeO3)2

Abstract

The angular dependences of the proton, deuteron, and ${\mathrm{Na}}^{23}$ magnetic-resonance spectra have been measured as a function of temperature in both normal (STSe) and deuterated (D-STSe) sodium trihydrogen selenite, and the electric field gradient tensors at the sodium and deuteron sites have been determined. These data show that the two ferroelectric transitions in STSe ($\frac{2}{m}\to 1\to m$) are accompanied by a doubling of the unit-cell edges in the $a$ and $b$ directions with respect to the paraelectric unit cell, whereas there is just one transition in D-STSe ($\frac{2}{m}\to m$), which is connected with a doubling in the $b$ direction only. The triclinic ferroelectric phase which exists in STSe between - 79 and - 173°C thus does not appear in D-STSe. The transitions are accompanied by an ordering of hydrogens (which in the paraelectric phase are rapidly jumping from one equilibrium site in the H bond to another) as well as by a distortion of the Se${\mathrm{O}}_3^{-}$ and ${\mathrm{Na}}^{+}$ network.