Ni3+ adsorbate dynamics on a NiO(001) surface
We present results concerning the dynamical behavior of a Ni3+ adsorbate on a NiO(001) surface obtained by molecular dynamics simulations. In a first place, we examined at low temperature the position of the Ni3+ ion as an adatom on the surface and the corresponding modification of its local environment as reflected on the pair-wise radial distribution function. The calculation of the vibrational properties of the adatom by means of the phonon local density of states (LDOS) shows that there is an anisotropic behavior both in the two principal in-plane directions as well as in the direction normal to the surface in accordance with the structural results. We compare the phonon LDOS of the Ni3+ adatom with the corresponding results for the Ni2+ adatom and the Ni2+ surface cations. Static energetic calculations are indicative that the exchange of the Ni3+ ion with a surface Ni2+ ion could be favorable. Such a behavior is confirmed by results observed at temperatures higher than 700 K where the Ni3+ adsorbate is located on a substitutional position on the surface and not on adatom position. The exchange takes place through simple or double exchange mechanisms. The structural and dynamical behavior of the Ni3+ ion at the substitution position was investigated in the temperature range 700-2000 K through the calculation of the pair distribution function, the relaxed interlayer relative position (RIRP), mean-square displacements (MSDs) and phonon LDOS. Results show that in comparison with the Ni2+ surface ions the Ni3+ ion at substitution position is more tightly bound especially in the direction normal to the surface as is indicated by the local structure and the contraction it presents as well as its phonon LDOS. As temperature increases the binding of the Ni3+ ion becomes less important as reflected on the physical properties mentioned above. (c) 2006 Elsevier B.V. All rights reserved.