RAMAN INVESTIGATION OF ROTATIONAL AND TRANSLATIONAL EXCITATIONS IN K1-X(NH4)XI MIXED-CRYSTALS

Using Raman scattering experiments we have investigated the rotational and translational modes in K1-x(NH4)xI mixed crystals for ammonium co ncentrations x = 0.005, 0.07, 0.16, and x = 0.46 and in a fully deuter ated isomorph with x(D) = 0.42. The A1g + 4E(g) and T2g Raman response s have been measured at atmospheric pressure in the paraelectric plast ic phase (T = 295 K) and in the orientational glassy phase (T = 7 K). In the low-frequency region nu = 5-600 cm-1, seven Raman active compon ents could be identified. These bands are entirely due to the admixtur e of anisotropic NH4+ to pure KI. Their contributions result either fr om reorientations of the molecules between equivalent directions and r otational-like excitations around these preferential orientations, or from translational modes of the whole Brillouin zone. In this peculiar case, orientational disorder of the ammonium lifts wave vector conser vation of the scattering processes. An interpretation of the different Raman allowed transitions is proposed. The composition and polarizati on dependences as well as the effect of deuteration for these modes ar e discussed with respect to earlier reports.