X-Ray Study of Phase Transition in NaNO 3

The phase transition of NaNO 3 crystal associated with the ordering of the orientation of NO 3 - ion has been investigated by X-ray scattering. The experimental results have been analyzed on a microscopic basis to obtain pair interactions between NO 3 - ions. The results are summarized as follows. (i) The ellipsoidal distribution of critical diffuse scattering in the reciprocal space is explained by taking the pair interactions up to second nearest neighbours. (ii) The temperature dependence of critical scattering gives a reasonable agreement with theoretical values in random phase approximation in the temperature range of T > T c +4.5 K. (iii) Both the temperature dependence of long range order parameter and of anomalous lattice expansion can be satisfactorily explained by taking the strain dependence of pair interaction energy into account.