Theoretical calculations on the structures, electronic and magnetic properties of binary 3d transition metal nitrides

The electronic structures of a number of binary 3d transition metal and iron nitrides, some of which still need to be synthesized, have been investigated by means of spin-polarized first principles band structure calculations (TB-LMTO-ASA). The chemical bonding in all compounds has been clarified in detail through the analysis of total and local densities-of-stales (DOS) and crystal orbital Hamilton populations (COHP). The binary transition metal nitride set includes ScN, TIN, VN, CrN, MnN, FeN, CoN and NiN, both in the sodium chloride as well as in the zinc blende structure type. Antibonding metal-metal interactions for higher electron counts are significantly weaker in the zinc blende type, thus favoring this structural alternative for the later transition metal nitrides. For binary iron nitrides, the stoichiometric phases alpha "-Fe16N2, gamma'-Fe4N, epsilon-Fe3N, zeta-Fe2N as well as the recently synthesized (rf-sputtering) non-stoichiometric compounds gamma "-FeN0.91 and gamma"'-FeN0.5-0.7 have been investigated. There is experimental evidence that gamma "-FeN0.91 adopts the zinc blende structure type while gamma'''-FeN0.5-0.7 should crystallize in a defect sodium chloride type structure. For the stoichiometric phases; most numerical theoretical data are consistent with the measured groundstate properties (lattice parameters and magnetic moments) whenever experimentally available. The general trends concerning iron-nitrogen and iron-iron bonding have been elucidated; the role of nitrogen vacancies were simulated by a number of model calculations. It appears that potentially antibonding interactions are the source of local structural distortions in all of these phases. For gamma "-FeN0.91, theory supports the proposed metallic zinc blende structure with a theoretical lattice parameter of 421 pm for the exact 1 : 1 composition. With respect to gamma'"-FeN0.5-0.7 in the defect NaCl structure type, we arrive at theoretical lattice constants between 389 and 398 pm, somewhat depending upon the nitrogen content.