Some Experimental and Theoretical Studies of Ferroelectricity in NaNO₂

Abstract

The dielectric constant of NaNO₂ crystal was measured in a temperature range from r.t. to 200°C and in a frequency range from 60 to 10⁶ 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/cm² at r.t.. The ferroelectricity in NaNO₂ 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−T₀)P²+BP⁴+CP⁶, 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 T₀=436.2°K. From the dielectric measurements, it is concluded that the transition in NaNO₂ 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 NaNO₂ crystal is a ferroelectric crystal which stands halfway between ionic and molecular crystal.