AB-INITIO PHONON-DISPERSION CURVES AND INTERATOMIC FORCE-CONSTANTS OFBARIUM-TITANATE

The phonon dispersion curves of cubic BaTiO3 have been computed within a first-principles approach and the results compared to the experimen tal data. The curves obtained are very similar to those reported for K NbO3 by Yu and Krakauer (Phys. Rev. Lett., 74, 4067 (1995)). They reve al that correlated atomic displacements along [100] chains are at the origin of the ferroelectric instability. A simplified model illustrate s that spontaneous collective displacements will occur when a dozen of aligned atoms are coupled. The longitudinal interatomic force constan t between nearest neighbour Ti and O atoms is relatively weak in compa rison to that between Ti atoms in adjacent cells. The small coupling b etween Ti and O displacements seems however necessary to reproduce a l inear ferroelectric instability.