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

Results of full-potential linear-muffin-tin-orbital calculations for the new intercalated niobium oxide phases LixNbO2 and NaxNbO2 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 M1.0NbO2 crystals making them semiconducting. In the nonstoichiometric MxNbO2 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 Tc in the scope of the rigid-ion approximation result in Tc∼12 K for Li0.5NbO2 and Tc∼9 K for NaxNbO2 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 Li0.5TiO2. It is shown that the density of states at the Fermi level and Tc for NaxNbO2 should be more critically dependent on stoichiometry than for LixNbO2. 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-Tc superconductors should be expected in this class of compounds.