Diffuse scattering and ordering in the short-range modulated paraelectric phase of sodium nitrite, NaNO₂

The incommensurately modulated antiferroelectric phase of sodium nitrite, NaNO₂, transforms at T_N = 437.7 K to the short-range modulated paraelectric phase. The apparently discontinuous phase transition is accompanied by characteristic changes in the diffraction pattern. Contrary to the well known modulated structures with sharp satellite reflections, the diffraction pattern of a short-range modulated structure contains diffuse satellite reflections. The short-range modulated crystal structure of the paraelectric phase of sodium nitrite has been analysed by the Reverse Monte Carlo (RMC) simulation of X-ray diffuse scattering. The crystal structure of sodium nitrite may be regarded as consisting of [Na⁺NO]_∞ rows running along the polar b axis. One can expect long fragments of rows with uniform polarity The assumption that single [Na⁺NO]_∞ rows are polar with uniform polarity proved to be a convenient approximation which is in good agreement with the observed diffraction pattern. The distribution of (+)- and (−)-[Na⁺NO]_∞ polar rows crossing the (010) plane of short-range modulated NaNO₂ revealed by RMC shows nanodomains consisting of distorted fragments of a sinusoidally modulated crystal structure. The size of the nanodomains and the degree of order in paraelectric NaNO₂ decreases with temperature.