Anion Rotational Disorder in Sodium Nitrate

The photographic study of X-ray diffuse scattering from a single crystal was made in order to determine the rotational disorder of NO 3 groups above the critical temperature 275°C. The rounded diffuse maxima which appear at the positions of super-lattice reflections of the ordered lattice are identified with those due to the disorder of NO 3 groups. It is concluded that the rotation of NO 3 group is hindered and the NO 3 group is almost bound in two positions which have the calcite-type of coordination with the surrounding Na atoms. There exists such a short-range order that the neighboring NO 3 groups have a tendency to take the ordered arrangement.