Theoretical study of the surface structures and vibrations of solid α-N2 a)

Abstract

The structures and vibrations of the (001), (110), and (111) surfaces of solid α-N2 at zero temperature have been studied using the modified Thiéry–Chandrasekharan potential model. The equilibrium structures of the semi-infinite surfaces are determined using a static energy minimization scheme, and the Einstein frequencies for both translational vibrations and librations of the molecules near the surfaces, and their root mean square (rms) oscillation amplitudes, are calculated. Based upon the equilibrium surface structures, the lattice dynamical calculations for finite thickness slabs [20 layers for the (001) and (110) surfaces and 10 layers for the (111) surfaces] have also been made for wave vectors along certain paths in the two-dimensional Brillouin zones. It is found that the molecular centers of mass positions, orientations, and bond lengths for the molecules in the vicinity of the surfaces, as well as the Einstein frequencies and rms oscillation amplitudes, are significantly different from their bulk values. In the lattice dynamics of the slabs, various types of surface modes are observed. They are (1) surface phonon modes, (2) surface librational modes, (3) hybrid modes of surface phonons and librations, and (4) surface intramolecular vibrational modes. The first two types of the surface modes can also have bulk librational and bulk phonon components, respectively. Furthermore, mixed modes between surface and bulk modes are observed for each of these four types of modes. The influence of the zero-point vibrations on the surface structures and the surface modes is estimated.