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 concentrations x = 0.005, 0.07, 0.16, and x = 0.46 and in a fully deuterated isomorph with x(D) = 0.42. The A1g + 4E(g) and T2g Raman responses have been measured at atmospheric pressure in the paraelectric plastic 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 components could be identified. These bands are entirely due to the admixture of anisotropic NH4+ to pure KI. Their contributions result either from reorientations of the molecules between equivalent directions and rotational-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 conservation of the scattering processes. An interpretation of the different Raman allowed transitions is proposed. The composition and polarization dependences as well as the effect of deuteration for these modes are discussed with respect to earlier reports.