Critical Fluctuations in the Paraelectric Phase of NaNO2

Molecular-dynamics calculations have been used to investigate the nature of critical fluctuations in the paraelectric phase of NaN${\mathrm{O}}_2$. Although the ${\left(\mathrm{N}{\mathrm{O}}_2\right)}^{-}$ anions are found to reorient rapidly, predominantly about the crystal $c$ axis, the simulation reveals the existence of long-lived fluctuations of antiferroelectric character. The reorientational motion is found to couple to optic rather than acoustic modes, in agreement with a recent theoretical prediction.