Is NaNO2 a pure order–disorder type ferroelectric?

X-ray analysis of the displacement parameters Uii has been performed for one of the most typical order–disorder type ferroelectrics, NaNO2, below and above Tc. It is strongly suggested that the thermal behavior of the Na atom has a displacive character and combining this result with evident disorder of the NO2 ion leads to the conclusion that the transition is of mixed type, involving both of order–disorder and displacement. It is emphasized that the Na atom plays an important role and that the coupling between the disordering of the NO2 ion and the translational motion of the Na atom leads to the phase transition. Similar conclusion is also drawn for the other typical ferroelectric KH2PO4.