Lattice dynamics of α-CO from an a b i n i t i o potential

Abstract

Harmonic lattice dynamics calculations have been performed for the antiferroelectric P213 ordered phase of α-carbon monoxide and for two related structures with a different head–tail ordering of the CO molecules. The potential used is an ab initio CO–CO potential with its anisotropy expanded in spherical harmonics, as well as a site–site model fitted to this ab initio potential. Good agreement with experiment is obtained for the structure, the cohesion energy, and the phonon frequencies without any adjustment of the potential. Head–tail reordering of the molecules is energetically almost neutral if it is accompanied by a translational displacement along the bond axes, which is symmetric with respect to the inversion centers of the average Pa3 structure. This displacement is induced in particular by the strong head–tail anisotropy in the short-range repulsion. Translation–rotation coupling is found to be important; it affects especially the frequency of the lowest optical mode.