Infrared Absorption Properties of LiH, LiD Mixed Crystals

The normal modes of pure LiH and LiD, required in impurity mode calculations, have been computed on the basis of the deformation-dipole model. Neutron-scattering results of Verble, Warren, and Yarnell for ${\mathrm{Li}}^7$D, along with some other experimental quantities, have been used to determine the various parameters involved. It is found that the frequency spectrum of LiH has a gap and that a substitutional ${\mathrm{D}}^{-}$ ion produces a local mode in this gap whose frequency has been computed, together with its amplitude at the impurity. The infrared absorption frequency associated with this local mode and its integrated absorption have been computed as functions of ${\mathrm{D}}^{-}$ content in the limit of low ${\mathrm{D}}^{-}$ concentration. Also, we find an in-band resonance in the amplitude of the substitutional ${\mathrm{D}}^{-}$ ion. Similar calculations have been carried out for ${\mathrm{H}}^{-}$ impurities in LiD. The present results are compared with those of other calculations and with experiment.