a Behavior-Type Raman Study of Orientationally Degenerate Defects in Doped Alkali Halides.

Localized defects with unusual rotational or translational motion in alkali halides are investigated by polarized Raman scattering. Molecular impurities reorient between several equivalent equilibrium orientations. The KI:Ag ^{+} system occupies a regular lattice site at 2K and goes off-center, when the temperature is raised to 25K. The free electron in the heavy-metal dimer centers is moving between two equivalent configurations. These motions influence the composition of the Raman spectra, their temperature dependence, and their polarization properties. Raman data represented for several diatomic (CN^{-}, SH^{-}, O_sp {2}{-}, and OD^ {-}) and triatomic (NCO^ {-}) linear molecules, and for the bent triatomic NO_sp{2}{-} molecular ion in KCl, KBr, KI, and NaBr. Preliminary results for KI_{1-x}(NH _4)I are also reported. The symmetry assignment by polarized Raman scattering is in agreement with previous results for the diatomic systems. The C _{3v} symmetry of the static NCO^{-} in KBr is established for the first time. Surprising and new results are obtained for the symmetries of NO_sp{2} {-} and NH_sp{4} {+}. The influence of rotational motion and polarizability of the surrounding ions on the polarized Raman data is discussed. New and different polarized Raman behavior is established for mixed molecular crystals of higher concentrations, reflecting electric/elastic dipole interactions. The on- to off-center transition of the KI:Ag^+ system is tracked by the temperature dependence of the Raman scattering data in the range between 2 and 25K. Calculations for the on-center configuration indicate that the temperature behavior is in contrast with that of 'soft-mode' systems. A possible mechanism, involving localized anharmonic modes trapped at the impurity site, is discussed and correlated with the unusual polarization properties of the Raman scattering. Thermally activated reorientation and superionic motion are compared. The resonant Raman data for Tl^+Tl ^0(1)-like dimer centers in Ga^+ - and In^+-doped alkali halides are presented. The Ga^+Ga ^0(1) (In^+In ^0(1)) defects in KCl and RbCl are identified. The correspondence with previously suggested models is discussed. The Raman response of the dimer centers is determined by the non-adiabatic motion of the unpaired electron. The non -atomic behavior of the third excited state of Tl ^0(1)-like defects is shown to be reflected in the Raman data.