Temperature-induced orientational disorder inNaClO3

Abstract

A detailed study of the temperature dependence of ${}^{35}\mathrm{Cl}$ nuclear quadrupole spin-lattice relaxation time and resonance frequency, in a wide temperature range (77--450 K), is used to discuss old controversial conclusions about the experimental behavior of low frequency dielectric permittivity, Raman spectroscopy, and nuclear quadrupole resonance, in sodium chlorate. Compatibility among experimental data coming from all these techniques is achieved by assuming a continuous transition of the ${\mathrm{ClO}}_{3}^{\ensuremath{-}}$ ion dynamics from vibrating around an equilibrium position for $T<250$ K, to exchanging between two orientations at higher temperatures. That is, the pseudoharmonic single well potential, that determines the chlorate ion dynamics at lower temperatures, mutates continuously with increasing temperature, to reach a symmetric two-well potential for temperatures higher than 380 K. In the intermediate-temperature range the behavior of NQR parameters are explained in terms of scattering of phonons by a distribution of disordered chlorate ions. We use the Green function formalism for calculating the corrections to the spin-lattice relaxation time and the resonance frequency due to disorder.