Crystal Dynamics and the Ferroelectric Phase Transition of Sodium Nitrite

Abstract

The crystal dynamics of sodium nitrite has been studied by coherent neutron inelastic scattering techniques. The normal modes of vibration propagating along the three principal symmetry axes in the crystal at 296°K have been measured. The results have been used to find the parameters of rigid ion models for NaNO₂ in which the NO₂ ion was treated as a group. Although none of the models are entirely satisfactory they do give a fair description of the results. The temperature dependence of the frequencies of some of the normal modes was measured and none were found to be very temperature dependent near the ferroelectric transition. Ferroelectric critical scattering was observed which was quasi-elastic showing that the ferroelectric fluctuations change slowly compared with the frequencies of almost all the normal modes of vibration. A model is deduced for the ferroelectric fluctuations incorporating the motion of the atoms but it is found that the motion of the atoms in these fluctuations cannot be described by the same parameters as those used for the normal modes of vibration.