Vibrational Normal Modes of theD2h16 Phase of BaCeO3: a Critical Comparison of Force Fields

Abstract

Two independent calculations of vibrational normal modes of the D2h16 phase of BaCeO3 were performed. In one, the force constants were constrained to follow an arbitrary law vs. interatomic distances; in the other, the conventional strategy consisting in adjusting independently all the force constants was used. In the first case, the number of adjustable parameters determining the diagonal force constants is only eight. The force constants, the calculated wavenumbers, the cartesian displacements of atoms and the potential energy distribution are presented and compared for the two sets of data. The calculated Raman wavenumbers are roughly the same for the two sets. Most of the normal modes are similar in both calculations; the high wavenumber modes appear slightly more delocalized for set II. This comparative study of the two force fields shows that both sets give similar and reasonable agreement and hence that it is preferable to define a force field based on interatomic distance-dependent force constants: this leads to a substantial reduction in the number of adjustable parameters and allows them to be easily transferred to the different structural forms which are observed when the temperature is raised. The modes which becomes corner zone Raman inactive above the 290°C phase transition have been identified from their Lx cartesian atomic displacements. Some characteristic modes are represented either because they are able to be involved in phase transition or because they have a particular temperature or pressure dependence or because they differ from one calculation to the other. © 1997 by John Wiley & Sons, Ltd.