First-principles calculations of the vibrational properties of H centers in KCl crystals

The atomic and electronic structure of the radiation-induced interstitial halide atoms in KCl crystals (called H centers) is calculated using 16-atom supercells and the first-principles full-potential linear muffin-tin orbital method. The vibrational properties of the H centers are studied for the two orientations of the molecular axis of the H centers known to exist in alkali halide crystals — along the (110) and (111) axis. Only in the former case (which is known to be energetically more favorable in KCl), the local vibrational frequency is close to that experimentally observed by means of Raman spectroscopy. A strong coupling is found between the stretching molecular mode of the H center and the breathing motion of its six nearest (Formula presented) cations. This is predicted to result in another local mode around 257 (Formula presented).