STRESS DEPENDENCE OF THE POCKET GAP MODES IN KI-AG+

Far-ir uniaxial-stress experiments reveal the unusual result that the pocket gap modes in KI:Ag+ couple much more to the fully symmetric (A1g) strain than to the tetragonal (E(g)) and trigonal (T2g) strains; other modes due to monatomic substitutional defects are found to couple with similar strengths to both A1g and E(g) strains. Theoretical calculations based on perturbed breathing-shell-model phonons and a simple anharmonicity model suggest the presence of unusual pressure-induced microscopic displacements in the vicinity of the defect's fourth-nearest neighbors. The coupling coefficients of the pocket gap mode at 86.2 cm-1 are measured to be temperature dependent, with the E(g) coefficient actually changing sign at 12 K; such behavior has not been found for other impurity-induced localized modes.