Defect modes due to H? - D? pair impurities in KCl and KBr

Localised modes due to H^? - D^? pair impurities aligned in the 110] direction in KCl and KBr have been computed using a Green's function approach. The defect space used consists of 12 particles; the two defect ions (H^? and D^?) and their ten nearest neighbours (K⁺ ions). The system has C_2ν site symmetry around the defect ions, and this property has been used to block diagonalise the relevant 36 × 36 matrix by group theoretical techniques. Various irreducible representations thus obtained have been used to compute the defect modes. Numerical values of the various Green's functions computed on the basis of neutron-fitted shell model parameters were used. Mass changes as well as changes in the short range interactions were considered while forming the perturbation matrix. The localised modes computed are seen to be in good agreement with the experimentally observed values.