Light scattering by point defects and their complexes in ionic crystals

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Abstract
Mills' calculation (1980) of Rayleigh light scattering from isovalent impurities in an insulating crystal is generalised to be appreciable to aliovalent impurities, vacancies and their complexes in an ionic crystal of NaCl structure. Modelling the host crystal as an isotropic elastic continuum, three contributions to dielectric inhomogeneity are included: (i) the change in electronic polarisability at the defect site due to the defect, (ii) the elastic strain field around the defect due to ionic size mismatch, and (iii) the strain field arising due to effective charge on the defect. The authors find that the fourth power frequency ( omega 4) dependence of the scattering cross section sigma reported by Mills is still preserved. The magnitude of sigma is influenced by the interference between the mechanisms considered. In most systems, the net effect of mechanisms (ii) and (iii) is to reduce sigma by destructive interference. Estimates of sigma for a number of typical impurities in NaCl and KBr crystals show that the contribution due to the third mechanism exceeds that due to the second considerably. In the case of an impurity vacancy dipole, the Coulomb effects of the constituents cancel each other. The recent observation that Fe2+ impurity in a KBr crystal scatters much more strongly than Rb+ (in KBr) is explained on the basis of the present model.