X-Ray Study on the Phase Transition of NaNO₂

The precise temperature dependence of x-ray diffraction effects near the transition temperature of ferroelectric NaNO₂ crystal has been examined. The satellite reflections, 1±σ 5 0, start to appear at T_s, 0.2°C below the well-known first-order transition point T_t. Marked asymmetry of the satellite reflections was sometimes observed when a multi-domain crystal was measured. The period of the sinusoidal modulation along the a-axis varies from 10.3 a₀ to 8.4 a₀ with increasing temperature. The effect of a d. c. external field along the b-axis on the x-ray diffraction effect was also examined. The experimental evidence indicates the presence of a new phase between T_s and T_t. Some thermodynamical calculations for the phase transition of NaNO₂ were carried out. To explain the intensity asymmetry of the satellite reflections the diffraction effect for a distorted crystal was also calculated.