On the stability of ferromagnetism in intermediate valence systems

It is known that the deformations of the electronic charge density induced by lattice displacements play an important role in lattice dynamics. The multipole expansion of these deformations around the positions of both the cation and the anion leads to a rather general type of deformable shell model. This cluster deformation model can be applied not only to ionic crystals but also to a large variety of other materials like mixed-valence compounds. Taking TiN as an example, it is shown that the model is able to describe the very pronounced phonon anomalies in superconducting transitionmetal compounds and to reproduce the measured phonon dispersion curves within the experimental errors.