Disorder-Induced Light Scattering in α-AgI

Abstract

Raman results [1,2] on α-AgI, in the frequency range 7÷165 cm−1, show that both the integrated Raman intensity and the depolarization ratio R=Izx/Izz (which was also found to be constant versus frequency) are strongly temperature dependent, decreasing up to 450°C. In order to explain this unusual behaviour we proposed [3] a model, in which all of the observed light scattering is explained by a simplified charge — induced dipole mechanism (C.I.D.). In such a way it is possible to relate the two measured quantities with the arrangements of the mobile cations around each iodine ion. The observed temperature dependences were theoretically accounted for by introducing a correlation in the silver ion distribution, which is destroyed at increasing temperature. The spectral intensity was tentatively attributed to the vibrational dynamics of iodine atoms, while the motion of the silver ions has been neglected as source of light scattering in the frequency range measured. In the aim to clarify the problem of the scattering contributions at lower frequency we have calculated the integrated intensity as well as the depolarization ratio both from iodine vibrational dynamics and from silver motions [4]. The polarizability term due to the silver dynamics shows calculated values of AgR quite different from the ones related to the iodine term:AgR (180 ° C) =0 and AgR (450 °C) =1.5 [4], while the iodine contribution leads to IR (180°C) =1.05 and IR (450 °C) =0.69 [3]. Moreover, the measured depolarization ratio down to 2 cm−1 m presents no frequency dependence [4].