LATTICE-DYNAMICS OF LA2NIO4 IN THE CHARGE FLUCTUATION MODEL

The lattice dynamics of La2NiO4 is calculated in the framework of a de scription of the electronic density response recently applied to the h igh-temperature superconductors (HTSC). The local part of the density response is approximated by a proper ab initio ionic model of rigid io ns to represent the effects of the ionic forces in the crystal. The st ructure parameters and the phonon dispersion curves are investigated i n this model and are compared with recent interpretations of the exper imental data. Additionally, nonlocal contributions to the density resp onse in the form of charge fluctuations at the ions are considered and their influence on the dispersion is discussed. From our results we f ind that some corrections of the current interpretation of the experim ental data may be appropriate. Finally, the importance of phonon-induc ed charge fluctuations for the HTSC is emphasized.