Electronic structure and physical properties of early transition metal mononitrides: Density-functional theory LDA, GGA, and screened-exchange LDA FLAPW calculations

The desirable physical properties of hardness, high temperature stability, and conductivity make the early transition metal nitrides important materials for various technological applications. To learn more about the nature of these materials, first-principles density-functional theory calculations using the full-potential linearized augmented plane wave (FLAPW) method within the local-density approximation (LDA) and with the generalized gradient approximation (GGA) have been performed. We investigate the bulk electronic and physical properties of a series of early transition metal mononitrides, namely, those formed with 3d metals (ScN, TiN, VN), 4d metals (YN, ZrN, NbN), and 5d metals (LaN, HfN, TaN) in the rocksalt structure. In particular, lattice constants, bulk moduli, heats of formation, and cohesive energies as well as bulk band structures and densities of states are reported, and trends discussed. We find that the GGA yields 1%--2% larger lattice constants, 10%--20% smaller bulk moduli, and 10%--30% lower heats of formation compared to the LDA. The GGA slightly overestimates lattice constants, but leads overall to an improved agreement with experiment compared to the LDA, for which the values are too small. These materials are metallic with the exception of ScN, YN, and LaN, which appear to be semimetals within the LDA (and GGA), but are in fact semiconductors with indirect band gaps of 1.58, 0.85, and 0.75 eV, respectively, as revealed by calculations performed using the screened-exchange LDA approach. These last, relatively unexplored, refractory III-V nitrides may therefore have potential use in device applications; in particular, ScN is well lattice matched to GaN, a wide-band-gap semiconductor that is of great current interest in relation to optoelectronic devices, and high temperature and high power electronic applications.