Diffuse X-ray scattering and the anomalous soft mode in NaNO3

Triple-axis X-ray scattering measurements of diffuse scattering in NaNO 3 are described. NaNO 3 undergoes a structural phase transition at 549 K which results in the appearance of a superlattice reflection at the Z-point of the high-temperature Brillouin zone. However, strong diffuse scattering was observed in the low-temperature phase at (2.502), which is a zone boundary F-point of the high-temperature phase. The temperature dependence of the diffuse scattering was measured between 295 K and 538 K. The scattering was observed to take the form of a ridge between Gamma -points, and is linked with the partial softening of a transverse acoustic branch about the F-point. Using these results, an approximate determination of the magnitudes of the soft-mode eigenvectors was achieved. The observed behaviour is discussed in terms of the possibility of non-critical fluctuations into a monoclinic ordered structure, and is compared with the superficially similar behaviour of calcite (CaCO 3 ).