Defect modes due to substitutional anion- as well as cation-pair impurities aligned in the [110] direction in various alkali-halide crystals have been computed using the Green's function technique and compared with experimental results. The following systems were studied: KCl:${\mathrm{H}}^{-}$-${\mathrm{H}}^{-}$, KCl:${\mathrm{D}}^{-}$-${\mathrm{D}}^{-}$, KBr:${\mathrm{H}}^{-}$-${\mathrm{H}}^{-}$, KBr:${\mathrm{D}}^{-}$-${\mathrm{D}}^{-}$, RbCl:${\mathrm{H}}^{-}$-${\mathrm{H}}^{-}$, NaCl:${\mathrm{H}}^{-}$-${\mathrm{H}}^{-}$, KI:${\mathrm{H}}^{-}$-${\mathrm{H}}^{-}$, KI:${\mathrm{Cl}}^{-}$-${\mathrm{Cl}}^{-}$, KI:${\mathrm{Br}}^{-}$-${\mathrm{Br}}^{-}$, KI:${\mathrm{Na}}^{+}$-${\mathrm{Na}}^{+}$, KI:${\mathrm{Rb}}^{+}$-${\mathrm{Rb}}^{+}$, NaCl:${\mathrm{F}}^{-}$-${\mathrm{F}}^{-}$-${\mathrm{F}}^{-}$,NaCl:${\mathrm{Ag}}^{+}$-${\mathrm{Ag}}^{+}$, and KCl:${\mathrm{Na}}^{+}$-${\mathrm{Na}}^{+}$. We have considered the vibrations of 12 particles, viz., the two-defect ions and their ten nearest neighbors; i.e., a 36×36-dimensional defect space has been used. Mass changes as well as the changes in the short-range interactions due to the introduction of defects have been taken into account. The system has $D_{2h}$ site symmetry around the defect ions which has been used to block-diagonalize the relevant 36×36 matrix by employing group theory. Six irreducible representations thus obtained were used to compute the defect modes, three of which are infrared active while the remaining three are Raman active. Numerical values of the required Green's functions calculated on the basis of the neutron-fitted shell-model parameters were used. The computed modes are found to be in good agreement with the experimental values.

• Received 23 August 1979

DOI:https://doi.org/10.1103/PhysRevB.21.4850