ELECTRIC-FIELD DEPENDENCE OF POCKET AND NONPOCKET IMPURITY GAP MODES IN KI AND THE ESTABLISHMENT OF AG+ QUADRUPOLAR DEFORMABILITY

Far-infrared static electric field measurements have been made for a v ariety of KI point defects which produce vibrational modes in the pure crystal phonon gap. The extremely small field-induced frequency shift s (less than or equal to 0.02 cm(-1)) of the KI gap modes associated w ith anion impurities were accurately determined with a precision of +/ -0.003 cm(-1) by using a global analysis method. No frequency shifts w ere observed for the Rb+ or Cs+ gap modes, up to the maximum applied f ield of similar to 100 kV/cm in the [100] direction. Most revealing ar e the field-induced frequency shifts for the pocket gap modes associat ed with the Ag+ impurity, which are nearly two orders of magnitude sma ller than the field-induced shifts measured for low-frequency Ag+-indu ced resonant modes. The fact that the pocket-mode displacements are sh arply peaked on the (200) family of ions renders them sensitive to the host-lattice anharmonicity near those sites, whereas the resonant mod es probe the defect and its nearest neighbors. These E-field measureme nts and earlier stress-shift measurements are analyzed using a quasiha rmonic perturbed shell model. In this approach the effect of either an applied stress or an applied E field is to move the equilibrium posit ions of the ions, thereby renormalizing the harmonic force constants v ia the local cubic and quartic anharmonicity. The two types of experim ents produce local strains of orthogonal symmetries, and hence provide complementary information. The theoretical analysis of the E field an d stress measurements allows us to establish firmly that the Ag+ ion i n KI possesses a significant electronic quadrupolar deformability. In turn, this finding strongly supports earlier suggestions that the silv er ion quadrupolar deformability is an important feature in the dynami cs of other host-silver defect systems and of the silver halides.