Electronic and structural properties of ${\mathrm{La}}_3$${\mathrm{Ni}}_2$${\mathrm{B}}_2$${\mathrm{N}}_3$

${\mathrm{La}}_3$ ${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$ ${\mathrm{N}}_3$ is determined, using local-density-approximation calculations, to be a three-dimensional metal, similar to ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$C and related borocarbides. The band structure shows four hybridized bands crossing the Fermi energy, ${\mathit{E}}_{\mathit{F}}$. The Fermi-surface velocities in the x and z directions are 2.9×${10}^7$ cm/s and 1.5×${10}^7$ cm/s, respectively, i.e., a 2:1 ratio. This three-dimensional character is due to the open-shell character of the La ions and resulting participation of La orbitals in the states at ${\mathit{E}}_{\mathit{F}}$. The dominant La contribution at ${\mathit{E}}_{\mathit{F}}$ comes from La 5d orbitals, particularly for the outer La layers; La 4f contributions are weaker, but significant. There is a pronounced peak in the density of states occurring near ${\mathit{E}}_{\mathit{F}}$ due to a flat band, though this peak is somewhat smeared on the high-energy side as compared to the corresponding peak in ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$C. Internal structural coordinates and the corresponding optical Raman phonon frequencies have been calculated using force methods.