X-ray study on the evolution of thermal motion in the ferroelectric phase of ${\mathrm{NaNO}}_2$

The crystal structure of ${\mathrm{NaNO}}_2$ has been refined in the ferroelectric phase at 338, 378, and 418 K. The evolution of the thermal motion with increasing temperature has been studied by comparing the anisotropic displacement parameters, including earlier values at low temperatures. The results support the rotation of ${\mathrm{NO}}_2$ around the c axis as the mechanism of polarization reversal. It is strongly suggested that the coupling between the translational motion along the b axis and the rotational motion of the ${\mathrm{NO}}_2$ ion becomes stronger approaching to $T_c,$ reducing the effective potential barrier for ${\mathrm{NO}}_2$ rotation. The location of the rotation center is estimated to be slightly closer to the N atom. The electron density contribution from a small amount of disordered atoms has been found.