Tungsten atoms and clusters adsorbed on the MgO(001) surface: A density functional study

Adsorption of tungsten clusters W-n (n = 1-4) on the ideal MgO(001) surface has been studied computationally using a scalar relativistic density funct ional method and a gradient-corrected exchange-correlation functional. Stru cture and energetic features of the adsorption complexes W-n/MgO(001) have been analyzed. The oxide surface was represented by cluster models embedded in a large array of point charges (PCs). To reduce the artificial polariza tion of oxygen anions in the immediate vicinity of positive PCs, the cation s at the cluster boundaries were treated as Mg2+ ions at either all-electro n or pseudopotential (PP) level. Compared to the all-electron + PC embeddin g, the significantly more economic PP + PC approach is demonstrated to impo se cluster model boundary conditions appropriate to the ionic oxide MgO. Th e cluster size dependence of the adsorption properties is found weak. Like other transition metal clusters considered previously, tungsten species fav or adsorption sites in the proximity of oxygen centers of MgO(001). Rather small calculated adsorption-induced deformations of the tungsten clusters m anifest notably stronger W-W bonds compared to W-O bonds between metal and substrate. The tetrahedron shape of W-4, most Stable in the gas phase, is c alculated to be energetically preferred also in the adsorbed state, in part icular over a square-planar adsorbate. This finding is at variance with a m odel of two-dimensional W-4 clusters on MgO(001) derived from a recent high -resolution electron microscopy investigation (Tanaka, N., et al. Surf: Rev . Lett. 1998, 5, 723). The configuration of W-3 With two W atoms located cl ose to two nearest-neighbor oxygen ions is favored over that where two W at oms are close to next-nearest-neighbor substrate anions. In both cases, the adsorbed W3 cluster tilts considerably from an upright orientation.