Displacive modulation in the sinusoidal antiferroelectric phase of NaNO2.

Abstract

The structure of the incommensurate phase of ${\mathrm{NaNo}}_{2}$ at 162.5 \ifmmode^\circ\else\textdegree\fi{}C is elucidated using x-ray data collected by B\"ohm [Z. Kristallogr. 148, 207 (1978)]. The analysis is based on the four-dimensional super-space-group ${P}_{1\ifmmode\bar\else\textasciimacron\fi{}s1}^{\mathrm{Immm}}$, and shows that the sinusoidal modulation of the dipole moment of ${\mathrm{NO}}_{2}$ is accompanied by displacive modulation. The displacement wave is a transverse acoustic mode with the wave vector k\ensuremath{\rightarrow}\ensuremath{\sim}0.1a${\ensuremath{\rightarrow}}^{\mathrm{*}}$, the amplitude of which is parallel to the b axis. This is in phase with the modulation wave of the dipole moment in accordance with theoretical expectation.