The effect of second and fourth nearest neighbours on impurity induced Raman scattering

The E_g component of Raman scattering from KBr: Tl and KCl:Tl at 75°K is calculated and fitted to experimental data. The calcualtion is based on a Breathing Shell Model the parameters of which are determined by best fit to neutron scattering data. Three scattering models are compared. (1) With polarizability change produced by nearest neighbours (nn) only. In this case no fitting parameters are necessary. (2) With nn and 2nd nn. Where, two fitting parameters are needed. (3) With nn and 4th nn. Here, only one parameter is needed. Comparison of the different theoretical results for the scattering spectra shows, (a) that the inclusion of 2nd or 4th nn greatly improve the fit between theory and experiment; (b) that the polarizability parameters associated with 2nd nn and 4th nn are close in magnitude to that due to nn, and (c) that the inclusion of 2nd nn improves the high energy end of the spectrum but distorts the low energy end, whereas the inclusion of 4th nn only somewhat distorts the low energy end but the fit at the high energy end is less satisfactory. It is concluded that additional experiments may be necessary to decide which of the last two models is more appropriate.