Far-infrared and Raman studies of the incommensurate structureRb2ZnBr4. (A superspace approach)

The temperature dependence of the optical vibration spectrum of the modulated structure ${\mathrm{Rb}}_2$Zn${\mathrm{Br}}_4$ has been studied with Raman and far-infrared (FIR) spectroscopy in the normal, incommensurate, and different commensurate phases. The observed activation of new modes confirms the predicted change in selection rules as derived from superspace symmetry. A physical interpretation of the results is given by means of a harmonic oscillator model with modulated spring constants in which the superspace translation symmetry of the vibration modes is taken into account. The FIR response of such a system has been derived. At lower temperatures probably three commensurate phases seem to appear which are driven by a temperature-dependent modulation amplitude.