X-ray study of the phase transition of NaNO2

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Abstract
NaNO2 is known to transform from the ferroelectric room temperature phase to the paraelectric high temperature phase at 163°C. At this transformation an intermediate state is formed which has a modulated structure showing satellite reflections in the x-ray diffraction pattern.1 The temperature range in which the modulated state exists depends on the thermal history of the crystal. For a virgin crystal grown from aqueous solution this range is about 1 degree; for a crystal which was heat-treated for several weeks at the transformation temperature the range has extended to 12 degrees. For this heat-treated crystal the modulation of the structure occurs in a large temperature range (about 11 degrees) where the average structure is non-centrosymmetric and in a smaller temperature range (1 degree) where the average structure is centrosymmetric. Subsequent heat treatment of such a crystal in an external electric field reduces the temperature range for the modulated state considerably. The dependence of the temperature range of the modulated state upon the thermal history of the crystal is assumed to be due to the formation of ferroelectric 180°-domains; the walls of such domains would induce the modulation of the structure. Thus, in the case of NaNO2 the course of the phase transformation from an ordered ferroelectric state to a disordered paraelectric state appears to be strongly dependent on the domain structure of the crystal.

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