Magnetic properties and bonding analyses of perovskite structure Co4N nitride have been investigated within density functional theory using both pseudo potential and all electron methods. In the same time, the structural and magnetic stability of pure cobalt in hexagonal close packed (HCP), face centered cubic (FCC) and body centered cubic (BCC) structures are reviewed. At equilibrium, non-spin polarized (NSP) and spin polarized (SP) calculations of the energy versus volume show that the ground state is ferromagnetic in both materials. HCP-Co is found to be more stable than the cubic ones. Magnetic moments of Co atoms in Co4N nitride respectively belonging to two different crystallographic sites are studied over a wide range of the cubic lattice constant, and a comparison with the FCC-cobalt one is given. The volume expansion in the nitride indicates that the corner Co I atoms show localized magnetism while face center Co II atoms exhibit an itinerant behavior. Like in FCC-Fe/Fe4N, a "low volume-low moment" and "large volume-high moment" behavior is observed for FCC-Co/Co4N. The density of states of the Co4N ferromagnetic ground state is interpreted within the rigid band model. The different bonding characters of Co I -N versus Co II -N are shown with help of electron localization fucntion ELF plots and spin resolved chemical bonding criteria.

We investigate atomic and electronic structures of the threading edge dislocations of GaN using selfconsistent-charge density-functional tight-binding approaches. Full-core, open-core, Ga-vacancy, and N-vacancy edge dislocations are fully relaxed in our total-energy scheme. The Ga-vacancy dislocation is the most stable in a wide range of Ga chemical potentials, whereas full-core and open-core dislocations are more stable than others in the Ga-rich region. Partial dehybridization takes place during the lattice relaxation near the dislocation in all cases. The dangling bonds at Ga atoms mostly contribute to the deep-gap states, whereas those at N atoms contribute to the valence-band tails. All the edge dislocations can act as deep trap centers, except the Ga-vacancy dislocation, which may act as an origin of yellow luminescence.

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In the framework of density functional theory (DFT), the structural and magnetic properties of FeN mono nitride have been investigated using the all electrons augmented spherical wave method (ASW) with a generalized gradient GGA functional for treating the effects of exchange and correlation. Calculation of the energy versus volume in hypothetic rocksalt (RS), zinc-blende (ZB) and wurtzite (W) types structures shows that the RS-type structure is more stable than the

An atomistic simulation of the threading ͑a + c͒-mixed dislocation core in wurtzite GaN has been carried out. Starting from models generated in the framework of continuum elasticity theory, two core configurations are obtained independently by using an empirical potential and a tight-binding based ab initio method. The most energetically favorable core with a 5 / 7-atoms ring structure is fully coordinated without wrong bonds, whereas the other with a complex double 5 / 6-atoms ring structure contains two rows of dangling bonds. Both core configurations introduce empty states spread over the upper half of the band gap.