Covalent mixing

After a survey of the basic theory and some experimental aspects of photoelectron spectroscopy which are relevant to actinide solids, two systems are illustrated elemental actinide metals, in which the Mott transition between plutonium and americium is evidenced in a photographic way by photoemission, and strongly ionic oxides, in which the 5f localized behaviour is clearly seen, and indications of f-p or d-p covalent mixing are investigated. 

Ouster molecular ccdculations, a number of which has appeared on actinide oxides , are very sensitive to covalent mixing of the actinide-oxygen external orbitals which has a local bonding character. This is emphasized by these calculations which take into account a finite number of shells around the bonded species. It is just possible that the cluster method has a tendency to overestimate covalency. 

In solids where cation-cation interaction is significant, by can be related to R, where R is the cation-cation separation. In cases where cation-anion-cation interaction is important, by is related to the covalent mixing parameter, X, of the cation-anion orbitals. For octahedrally coordinated cations, as in rocksalt and perovskite structures, the relevant mixing parameters are and in the following molecular wave functions... 

The intraatomic interactions, on the other hand, are stronger the weaker the covalent mixing, which makes the on-site coulomb energies... 

Fig. 1 Schematic molecular orbital diagram for Ti02 sixfold-coordinated T + ions, and threefold-coordinated ions with covalent mixing of Ti and O atomic states in SALC s. Symmetry designations for occupied valence band states, the empty conduction band states, and the 0-atom 2s shallow core state. Respective degeneracies for A, E and T states, are 1, 2 and 3 [13]...

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