Electronic band structure calculations of the MNX (M = Zr, Ti; X = Cl, Br, I) system and its superconducting member, Li-doped β-ZrNCl

Electronic band structure calculations have been performed for beta-ZrNCl, beta-ZrNBr, alpha-ZrNBr, alpha-ZrNl, and alpha-TiNX (X = Cl, Br, I) using the extended Huckel method. These calculations reveal that n-doped beta-ZrNCl has a single narrow band at the Fermi level which is stabilized by overlap of 4dz(2) orbitals on zirconium with respect to the remainder of the conduction band states, This band shows only a small degree of dispersion and can essentially be described as a localized Zr-Zr bond when the carrier concentrations are low. The calculated band structure is consistent with the experimental observations of activated carrier transport, and an indirect optical band gap in beta-ZrNCl and beta-ZrNBr, A simple picture is presented which describes a mechanism for strong electron-phonon coupling in the beta-ZrNX structure, suggesting that beta-LixZrNCl is a classical BCS superconductor. This localized 4dz(2) band at the Fermi level is absent in the electronic band structures of alpha-ZrNX and alpha-TiNX compounds. The dispersion relations for these compounds show a direct optical band gap, as observed experimentally. The absence of significant metal-metal bonding in alpha-MNX compounds implies that n-doping will be more difficult to achieve in compounds adopting this structure. The relative stability of alpha- and beta-polymorphs is also discussed. (C) 1998 Academic Press.