Electronic properties of stoichiometric Ba and O overlayers adsorbed on W(001)

In order to understand better the mechanisms responsible for the lowering of the work function that is observed when surface layers containing barium and oxygen are adsorbed on refractory metals such as tungsten, we have initiated a series of full-potential linearized augmented-plane-wave (FLAPW) electronic-structure calculations on well-defined model systems representing the Ba OE W interface. We report here our initial results for models in which c(2x2) monolayers of barium and oxygen have been positioned on both sides of a five-layer W(001) film, with the adsorbate atoms being placed above the fourfold-hollow sites of the tungsten surface. Two different adsorbate configurations have been investigated: ''tilted,'' where the adsorbed monolayers have been arranged so that the barium and oxygen atoms each cover a different fourfold-hollow site in the c(2x2) unit cell, and ''upright,'' where the overlayers are aligned vertically so that both adsorbates lie above the same site. We have minimized the total energy of the system to determine the optimal adsorbate positions within this set of configurations. We find that the calculated work function of the clean five-layer tungsten slab (4.65 eV) is lowered by approximately 2 eV by the adsorption of the barium and oxygen surface layers in either configuration, but the tilted structure has a significantly lower energy than does the upright. In addition, the position of the oxygen 2s state, which is very sensitive to the adsorbate geometry, strongly favors the tilted model. In both cases we find evidence of significant bonding between the d-like surface states of the tungsten substrate and both the Ba d and the oxygen 2p adsorbate levels. As a result, multiple dipoles are formed at the interface, and the competition between these polarized charge distributions leads to a net lowering of the work function