Molecular Model Calculations of Pair–Modes in Alkali Halides

The molecular model, developed by Jaswal for [100] aligned impurity pairs, has been extended to account for [110] aligned pairs and used to study U center (H−H− and D−D−) pairs in KCl and (Cl−Cl− and Na+Na+)-induced pair–modes in the gap of KI. In this model the impurity ions in the pair and their nearest neighbors are allowed to vibrate while the rest of the lattice is held fixed. For [110] and [100] aligned pairs, this gives 12- and 17-ion molecules respectively. The potential energy is treated in the rigid ion approximation with short range forces between the nearest neighbors only. After the single relevant parameter of the model, the nearest neighbor force constant, is fitted to the U center local mode frequency, the model can account for the experimentally observed frequencies and polarizations of the three U center pairs in KCl. Similarly, after fitting to the single ion Cl− gap mode at 77 cm−1 in KI, the model predicts 3 infrared active pair–modes whose frequencies are in close agreement with experiment. In KI:Na+, where no single ion resonance has been observed, the calculated results suggest that the absorption lines are due to [110] aligned Na+ ions, with a large softening of the short range force constants occurring around the defect. The KI:Br− and NaCl:F− systems are also considered.