Raman Scattering and Far-Infrared Absorption Induced by Silver Ions in Sodium Chloride

Abstract

A detailed experimental and theoretical study of the defect-induced far-infrared and first-order-Raman spectra of the NaCl: ${\mathrm{Ag}}^{+}$ system has been made. New low-temperature Raman data revealed many singularities pertaining to pure and perturbed phonons. Only a few of the singularities could be assigned to particular phonon critical points. The theoretical calculations were based on the so-called "lattice-relaxation" model, neutron-determined phonon parameters, and an accurate Brillouin-zone interpolation scheme. The calculated unperturbed $T_{2g}$ Raman spectrum fit the data well. The unperturbed $E_g$ spectrum gave a poor fit, primarily because it did not explain a very strong experimental $E_g$ peak at 96 ${\mathrm{cm}}^{-1\mathrm{}}$. This peak could be explained as an incipient resonance mode caused by relatively large decreases in the central-force constants. The infrared spectrum was fit well by assuming somewhat smaller central-force-constantdecreases. The $A_{1g}$ Raman spectrum could not be fit using force-constant decreases; it required an increase of the nearest-neighbor central-force constant. These results could be understood semiquantitatively as due to symmetry-dependent Coulomb contributions to force-constant changes resulting from lattice relaxation. The absolute scattering efficiencies and cross sections were determined for the three Raman symmetries and had $T_{2g}:A_{1g}:E_g$ intensity ratios of approximately 4: 10: 20.