N14NQR and relaxation at the ferroelectric transition in NaNO2

Abstract

${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ nuclear quadrupole resonance and relaxation measurements in NaN${\mathrm{O}}_2$ around the ferroelectric transitions have been performed by pulse and Fourier-transform technique, with high-temperature resolution and accuracy. In the narrow temperature range in which the antiferroelectric phase exists and where previous authors failed to detect the nuclear-quadrupole-resonance (NQR) signals, the splitting of the ${\nu }_{+}$ and ${\nu }_{-}$ lines in two components were observed. A theory is developed to connect the NQR parameters to the torsional vibrations of the N${\mathrm{O}}_2^{-}$ ions as well as to the order-disorder reorientational fluctuations driving the transitions. The critical fluctuations are described through a kinematical Ising model. It is proven that the reorientational order-disorder variable is reversed by an angle which is not exactly $\pi$. By analyzing the experimental results in the light of the theoretical treatment interesting information on the critical dynamics is obtained.