Another major subject, which has attracted much interest for the hexagonal types of SiC, is related to the electronic properties of extended defects, sur — faces/interfaces, stacking faults, and dislocations. The reason why extended defects have been mainly studied in the hexagonal types of silicon carbide lies in the fact that electronic properties of dislocations and stacking faults are particularly important for understanding the degradation of hexagonal SiC devices144 and the remarkable enhancement of dislo­cation velocity under illumination in the hexagonal phase.145 Nevertheless, some studies have been done for cubic SiC on the electronic structure of stacking faults146-151 and various types of dislocations.152-155 Obviously, a lot of work remains to be done on the extended defects in p SiC.

1.08.5.1 Uranium Oxide

1.08.5.2.1 Bulk electronic structure

Due to its technological importance and the com­plexity of its electronic structure, uranium oxide has become one of the test cases for beyond

LDA methods. Indeed, UO2 comes out as a metal when its electronic structure is calculated with LDA or GGA. This result has been found by many authors using many different codes or numer­ical schemes (the primary calculation being the work of Arko and coworkers156). The physical diffi­culty lies in the fact that UO2 is a Mott insulator. f electrons are indeed localized on uranium atoms and are not spread over the material as usual valence electrons are.

The first correction that has been applied is the LDA+U correction in which a Hubbard U term acting between f electrons is added ‘by hand’ to the Hamiltonian.157,158 This method allows the open­ing of an f-f gap.157 However, it suffers from the existence of multiple minima in the calculations, so the search for the real ground state is rather tricky as the calculation is easily trapped in metastable

159

states.

Hybrid functionals are another type of advanced methods that are very often used nowadays in the quantum chemistry community. Their principle is to mix a part of Hartree-Fock exact exchange with a DFT calculation; it has been applied to UO2 has been made by Kudin et a/.160 These methods are very promising for solid-state nuclear materials. However, the same problem of metastability as in LDA+U exists for such hybrid functionals,161 and the compu­tational load is much heavier than that in common or LDA+U calculations. Recently, an alternative to LDA+U has been proposed: the so-called local hybrid functional for correlated electrons162 in which the hybrid functional is applied only to the problematic f electrons. An application on UO2 is

available.163