Ellinger Y., Defranceschi M. (eds.) Strategies and applications in quantum chemistry (Kluwer, 200

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perpendicular direction. The selected clusters are obtained from the extension of along the a and c directions (model 2).

The (100) face is modelled by neutral clusters of general formula (MoO3)n (model 3), both

430 A. RAHMOUNI AND C. BARBIER

The (001) face has the highest density of Mo atoms; depending on the lattice fracture plane,

viz.

The density of states (DOS) of the infinite slab (Fig. 2) presents four blocks, two of them are located under the Fermi level (-14.58 eV); the analysis of orbital contributions to the

intense and unoccupied, the second at -15.3 eV is weak and occupied; orbitals form three bands: the most intense at -6 eV and two other very weak ones in an occupied region at -15.3 eV and -33 eV. Therefore, only the d bands located below -15 eV participate in a

The "Crystal Orbital Overlap Population" (COOP) [20] shows (Fig. 4) that all levels arising below the Fermi level are and bonding and the highest energy levels are and antibonding; however the specific COOP curves for each Mo-O distance (Fig. 5) show a

3. 2. (010) SURFACE CLUSTERS

two blocks around -15.2 and -33.2 eV of widths 1.3 and 1.7 eV respectively; the Fermi level is slightly removed and stabilized at -14.57 eV, the same value as in the band calculation.

In every case, the Mulliken population analysis displays a large electronic transfer from the molybdenum atom towards neighbouring oxygen atoms according to the ionic character of metal oxides. The charges on the molybdenum and oxygen atoms depend on the coordination number and also on the cluster size whether the latter is sufficient to reach the charge convergence or not. Indeed the latter is attained for a cluster containing 6 or 8 molybdenum atoms (Table 1).

the coordination number varies from 2 to 3 ; in other words the bonding capacity between oxygen and its nearest neighbours is shared between all bonds. Generally, the Mo-O overlap populations converge rapidly.

The infinite slab model can be seen as an infinite extension of the (010) clusters along cristallographic directions a and c, so one can compare limit values of (010) surface

models. The Mo-O overlap populations are generally in good agreement in both models, except for bonds (1.95 Å).

3.3.(001) AND (100) SURFACE CLUSTERS

Since molybdenum trioxide is built from distorted octahedrons, the atomic arrangement on each face is different. On the (010) cleavage plane each atom has the same coordination number as in the bulk whereas the other faces obtained by lattice breaking contain unsaturated atoms. The type of unsaturated atom depends on the face considered : unsaturated oxygen on the (001) face and unsaturated oxygen on the (100) face.

The evolution of electronic properties with increasing size of (001) clusters shows that whatever the face may be, convergence of most of the electronic properties is reached by a cluster containing 6 or 8 molybdenum atoms (Table 4).

The study of the surface cluster which models the (100) face shows that the creation of oxygen surface vacancies slightly increases the overlap population between unsaturated molybdenum and the nearest oxygen atoms (Table 5). In ionic compounds such as

one can consider that electron transfer takes place from so that the creation of the oxygen vacancies involves a decrease in the global electron transfer and consequently the molybdenum charge increases. On the other hand in order to compensate this decrease of electron transfer, the nearest remaining oxygen atoms supply more electrons to unsaturated molybdenum, this explains the decrease in the negative charge on the nearest oxygen and the increase in the overlap population between unsaturated molybdenum and the remaining neighbouring oxygen atoms.