Phase Transition in NaNO2

The temperature dependence of the X-ray diffraction intensity was precisely measured in order to clarify the nature of the ferroelectric phase transition in NaNO 2 . At the transition point, 163°C, the integrated intensity of the Bragg reflection decreases quite abruptly and distinct satellite peaks appear which are observed in a very narrow temperature range and which become diffuse with increasing temperature. It was concluded that the ferroelectric NaNO 2 crystal transforms through the first order transition to an antiferroelectric phase with a sinusoidal modulation of the moment in which the antiferroelectric order is partially attained. The system again meets with another transition to the paraelectric phase at certain temperature above the first transition point. A thermodynamic calculation on the basis of a molecular field approximation was performed. The successive appearance of three phases, the orders of transitions, the temperature dependence of the order parameter and the anomalous volume expansion are satisfactorily explained by this calculation.