Crystal structures with asymmetric and off-centre atoms

The conventional description of the physical behaviour of crystalline solids is based very largely upon the harmonic approximation, the validity of which is generally assumed to improve with decreasing temperature. For many years, only a few substances like ferroelectric BaTiO3 have appeared exceptions to this approach. During the last decade however, the application of modern physical techniques of analysis to some of the simpler crystallographic systems has revealed that low temperature anharmonicity is more common than had been realized. It has been established for example, that small impurity ions such as Li+ incorporated into KCl or KBr do not simply substitute for K+ but occupy off-centre positions and that this is quite directly a consequence of a potential well with a shallow maximum at the conventional lattice site together with the small size of Li+. Recent progress has been particularly stimulated by the discovery that similar anharmonic conditions may be of significance for high transition temperature superconductivity and for the magnetic behaviour of ferrite materials. Theoretically, many difficulties remain but the growing technological importance of this new field in physics seems likely to lead to increasing interest in the future.