Adsorption of transition metal atoms on oxygen vacancies and regular sitesof the MgO(001) surface

Adsorption of Cu, Ni, Ag, and Pd atoms on F-s and F-s(+) oxygen vacancy sit es as well as on regular O2- centers of the MgO(001) surface has been studi ed by means of gradient-corrected density functional calculations using clu ster models embedded in a matrix of model potentials and point charges, Sca lar relativistic effects have been taken into account for adsorbed Ag and P d species. The electronic structure, geometric parameters, and binding ener gies of the adsorption complexes have been calculated and analyzed with ref erence to the electronic properties of the vacancy sites and the metal atom s in question. For all adsorbates considered, adsorption is found to be str onger on F-s sites by 1-2.4 eV compared with regular O2- sites, with Pd and Ni forming the most stable complexes. On the F-s(+) site the single valenc e electron of Cu and Ag atoms couples with an unpaired electron of the vaca ncy forming a covalent bond. As a result the adsorption energy of these ato ms on F-s(+) is by more than 1 eV stronger than on the F-s sites; on the ot her hand, the adsorption energies of Ni and Pd are reduced on F-s(+) by 0.5 eV and 1.3 eV respectively. The whole series of M/F-s(+) complexes is char acterized by rather uniform values of adsorbate-substrate distances (1.5-1. 7 Angstrom) and adsorption energies (2.2-2.6 eV). (C) 1999 Elsevier Science B.V. All rights reserved.