Some Experimental and Theoretical Studies of Ferroelectricity in NaNO2

The dielectric constant of NaNO 2 crystal was measured in a temperature range from r.t. to 200°C and in a frequency range from 60 to 10 6 cycles per second. It shows peak at 164.2°C in heating and at 162.6°C in cooling. Dielectric dispersion, similar to the one observed in other ferroelectrics, exists in all directions of the crystal principal axes in the paraelectric region as well as in the ferroelectric one. The spontaneous polarization was determined by the measurement of the pyroelectric current and the value was estimated to be ca 8.6 µC/cm 2 at r.t.. The ferroelectricity in NaNO 2 was discussed in a phenomenological way, where the electrocaloric effect was taken into account. The free energy F was expressed as a function of the polarization P as F = A ( T - T 0 ) P 2 + B P 4 + C P 6 , and the coefficients were determined by the dielectric properties as A =1.22×10 -3 °K -1 , B =-0.3×10 -10 esu, C =1.0×10 -19 esu and T 0 =436.2°K. From the dielectric measurements, it is concluded that the transition in NaNO 2 is probably of the first kind. It was pointed out by the consideration about the atomic arrangement that the main contribution to the spontaneous polarization comes from the relative displacement between Na + and NO - 2 ions, and further that NaNO 2 crystal is a ferroelectric crystal which stands halfway between ionic and molecular crystal.