General theory of the effect of ionic motion on charge density

By analysing the first-order change in the total charge density of a system when one ion moves, a unique definition of a 'generalized' pseudoatom, involving both a rigid and a deformation part, is given. The dipole moment of this pseudoatom is related to the 'effective charge', which is shown to be the sum of two tensors, an isotropic one coming from the rigid part and an antisymmetric one coming from the deformation part of the pseudoatom. A generalized 'acoustic sum rule' is obtained. Force constants, pair-potential functions, bond-stretching and bond-bending effects are discussed in terms of the pseudoatom. The application of the theory to the perfect solid and its connection with standard theories involving the dielectric constant is made. The application of the theory to nearly-free-electron metals transition metals and covalently bonded semiconductors is discussed and the phonon spectra of diamond-type semiconductors is qualitatively explained.