Gap modes due to impurity pairs in ki

The pair modes due to Cl^?, Br^?, Na⁺W and Rb^v impurities in KI have been investigated using the Greens function technique. Group theory is used to put the final results as solutions of (2 × 2) determinants which could be tried by a hand calculator. Even a mass defect approximation reproduced the gap modes due to chlorine pairs, but in all cases a small change in the force constant, deduced from fitting single impurity gap modes, was used. The Raman active modes in all these cases were also computed.