Evidence for soft modes at {A figure is presented} ≠ 0 in the far-infrared spectrum of Rb2ZnBr4

Abstract

The possibility to observe optically inactive and {A figure is presented} ≠ 0 soft modes in an indirect way by using far-infrared techniques is investigated. Using the fact that, due to anharmonicity, all modes of lattice vibrations are coupled with each other, the softening of one particular mode can be seen indirectly in the damping of other modes. By applying this idea to the modulated structure Rb2ZnBr4, the infrared inactive soft mode at the normal-incommensurable transition, the soft phason mode at the lock-in transition as well as a soft mode behaviour at an intermediate temperature have been observed. © 1980.