Internal strain effects and the reduction of the number of ion-lattice interaction parameters

Abstract

The reduction in the number of ion-lattice interaction parameters, considered by several authors in the context of a uniform strain assumption, is generalised to systems under bulk stress or with long-wavelength acoustic phonon coupling, where the distortion is not necessarily one of uniform local strain. Many earlier simplifications are still valid, since the point group irreps appropriate to the symmetry coordinates affected are just those activated by a uniform strain field. The consequences of this for the dynamic crystal field and for the choice of a minimal set of coupling parameters are discussed, with particuLar reference to laCl, LaF3, and Cr2O3. Useful reductions are achieved when the number of ions in the coordination complex is equal to or greater than the number in the unit cell.