The elastic dipole tensor for point defects in ionic crystals

The author discusses the problem of calculating the elastic dipole tensor G for point defects in ionic crystals. It is shown that the relation between G and the derivative with respect to bulk strain of the defect formation energy Delta E provides a natural and practical means of calculating G. This relation may be exploited either by computing Delta E for a series of values of strain and extracting the derivative numerically, or by making use of an explicit expression for G in terms of the relaxed ionic positions. The method of calculation is illustrated for some colour centres for which experimental values of G are available: the H, V k and O 2 - centres in alkali halides and the V - centre in MgO. He obtains satisfactory agreement with experiment for the H and O 2 - centres, but rather poor agreement of the V k and V - centres. He discusses what may be learnt from the comparison between theory and experiment for G and suggests that further compositions of this kind may contribute to the improvement of atomistic models for point defects in ionic crystals.