COMMON INTERATOMIC POTENTIAL MODEL FOR THE LATTICE-DYNAMICS OF SEVERAL CUPRATES

A common empirical model is developed for the lattice dynamics of the cuprates La2CuO4, (La,Sr)(2)CuO4, Nd2CuO4, YBa2Cu3O6, and YBa2Cu3O7, w hich is also consistent with their crystal structures. Starting from t he idea that the binding in these compounds is primarily ionic in char acter, the interatomic interactions were modeled using shell models wi th short-range repulsive potentials of the Born-Mayer type. For the me tallic members of the series, screening of the ionic forces by free ca rriers was taken into account. There is clear evidence for an anisotro pic Cu-O potential whereas the polarizabilities could be kept isotropi c. The Cu(1)-O(4) bond in YBa2Cu3O6 could not be treated on a common f ooting, as there is too little charge on the Cu(l). The present study demonstrates that the lattice dynamics of all the compounds investigat ed can be described quite satisfactorily by a common model and it offe rs a basis for extensions to related compounds. On the other hand, we found clear indications that a description of the lattice vibrations w ith purely ionic forces has its limits for this class of compounds.