Dynamical properties of the Ni3Al low index surfaces with and without Ni or Al adatoms from molecular dynamics simulations

We present molecular dynamics simulation results on the vibrational properties of the Ni3Al low index surfaces with and without Ni/Al adatoms. We found that the mean square displacements of both surface atoms and adatoms depend linearly upon temperature, the adatoms on the (1 1 0) face exhibiting vibrational amplitudes twice as large as on the other two surfaces. In addition, we found that all surfaces studied are expanded and rumpled, the rippling effect being more pronounced in the case of the (0 0 1) face with the Al atoms occupying the topmost position. In the cases of the (1 1 0) and the (1 1 1) surfaces the rippling is visible at several atomic layers beneath the surface. The adatoms are found to be contracted at ambient conditions, an effect that is significant when deposited on the (1 1 0) face, attaining the value of -10%. Moreover, from the calculated phonon density of states (DOS) and phonon spectral densities at room temperature, we found that the presence of surfaces modifies the bulk phonon DOS, exhibiting new modes some of which are located in the bulk phonon gap. The present results are in line with available experimental and theoretical results