Electronic structure and electron-phonon interactions in layered LixNbO2 and NaxNbO2.

Abstract

Results of full-potential linear-muffin-tin-orbital calculations for the new intercalated niobium oxide phases ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ and ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ are presented. It is shown that alkali atoms are ionized, and their valence electrons completely fill the hybridized Nb d--O 2p conduction band in ${\mathit{M}}_{1.0}$${\mathrm{NbO}}_{2}$ crystals making them semiconducting. In the nonstoichiometric ${\mathit{M}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ phases, the Fermi level is located well inside the conduction band of predominant Nb 4d nature with significant contributions from O 2p states. Estimates of electron-phonon interaction parameters and ${\mathit{T}}_{\mathit{c}}$ in the scope of the rigid-ion approximation result in ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}12 K for ${\mathrm{Li}}_{0.5}$${\mathrm{NbO}}_{2}$ and ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}9 K for ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ in reasonable agreement with the experimental data---and show that superconductivity observed in these materials is of the usual electron-phonon type, similar to that in cubic ${\mathrm{Li}}_{0.5}$${\mathrm{TiO}}_{2}$. It is shown that the density of states at the Fermi level and ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ should be more critically dependent on stoichiometry than for ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$. Since the electronic-structure peculiarities obtained in the calculations are very different than those typical for superconducting cuprates, it does not appear likely that any high-${\mathit{T}}_{\mathit{c}}$ superconductors should be expected in this class of compounds.