Temperature dependence of the dielectric response of lithium niobate

Conditions are investigated for the occurrence of gap modes in diatomic linear chains with substitutional impurities and associated changes in force constants for the interactions between the impurity and its nearest neighbours and between first and second neighbours. Particular attention is given to `pocketʹ modes in which displacements of the second neighbours are dominant. The results are obtained by Greenʹs function analysis guided by direct calculations with finite chains. Comparisons are made with existing observations of gap and pocket modes in KI with substitutional impurities.