FIRST-PRINCIPLES DETERMINATION OF THE SOFT MODE IN CUBIC ZRO2

A direct approach to calculate the phonon dispersion using an ab initi o force constant method is introduced. The phonon dispersion and struc tural instability of cubic ZrO2 are found using a supercell method in the local-density approximation. The force constants are determined fr om the Hellmann-Feynman forces induced by the displacement of an atom in the 2 x 2 x 2 fcc supercell. This size of the supercell gives ''exa ct'' phonon frequencies at Gamma,X,L,W Brillouin zone points. The phon on dispersion curves show a pronounced soft mode at the X point, in ag reement with the experimentally observed cubic to tetragonal phase tra nsition.