Calculation on Vk-Centres in Alkali Halides

A simple model for studying the stability of V k -centres in alkali halides is proposed. The lattice energy is replaced by a Hooke's law type expression with three force constants (central forces only) between the nearest alkali-alkali, alkali-halogen and halogen-halogen ions. The electron-lattice coupling is taken into account by two components which are the first derivatives, with respect to the distance between the ions, of self-energy and transfer energy (between the nearest neighbour halogen orbitals) of the hole. Conditions for the V k -centre to be more stable than one center trapped state are obtained for different cases. On the other hand the V k -centre molecular vibration frequency and the optical absorption energies are evaluated from the known values of such quantities as macroscopic elastic constants and valence band widths. They are in order of magnitude agreement with experimental data. A few experiments are suggested to check some of the results obtained regarding the variation of V k -centre vibration frequency and the U V absorption energy for a given species of halogen X 2 - .