Raman scattering and local force variations due to transition-element impurities in zinc-sulfide crystals: Effect of pressure application

The results of Raman scattering experiments in zinc-sulfide containing transition-element substitutional impurities (Cr, Mn, Fe, Co, Ni) are reported at different pressures up to 40 kbar. Numerical calculations of these impurity-induced Raman intensities are also made on the basis of Green's-function theory by applying a very simple model of the impurity-host bonding and lattice phonons from an eleven-parameter rigid-ion model (RIM11). The theoretical analysis is made possible by the recent determination of a set of force parameters in terms of RIM11 for any pressure. The calculated results provide very good correspondence with all the experimental data. It is shown that the features observed in the Raman spectra of the above crystals may be interpreted as a series of resonant modes due to the weakening of the impurity-sulfur bonds by about 20%. It is demonstrated that pressure application is a very useful tool in the assignment of the defect modes, and that a calculation of the entire defect Raman spectrum is important for an unambiguous evaluation of the local force variations. Furthermore, the advantage of simultaneously dealing with a series of similar impurities is pointed out. A monotonic evolution of the local force variations is found when going through the series of impurities, which is tentatively correlated with the variable number of the $3d$ electrons contributing to the impurity-sulfur orbitals.