Electronic structure and crystal chemistry of niobium oxide phases.

Abstract

Results of local-density linear muffin-tin orbital (LMTO) electronic-structure calculations for both known and recently obtained niobium oxide phases with different crystal lattices and different coordination of Nb atoms are presented. Perovskite-like compounds with one- or two-dimensional Nb-O condensed clusters, as well as hexagonal layered structures with trigonal-prismatic coordination of Nb atoms, are used. For ${\mathrm{SrNbO}}_{3}$, ${\mathrm{BaNb}}_{5}$${\mathrm{O}}_{8}$, ${\mathrm{BaNb}}_{4}$${\mathrm{O}}_{6}$, and ${\mathrm{Sr}}_{2}$${\mathrm{Nb}}_{5}$${\mathrm{O}}_{9}$, the Fermi level is located at the shoulder of the Nb d conduction band with a low density of oxygen states. For hexagonal ${\mathit{M}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ (M=Li,Na) compounds, oxygen contributions definitely appear at ${\mathit{E}}_{\mathit{F}}$, but remain small compared with those of the high-${\mathit{T}}_{\mathit{c}}$ superconducting cuprates. Pair-interaction energies and superexchange-interaction parameters were calculated using the LMTO Green-function method. Typical for high-${\mathit{T}}_{\mathit{c}}$ superconductors, antiferromagnetic coupling between metal atoms in the Nb-O planes is shown to exist, but not the ferromagnetic exchange between metal d and oxygen p states. The calculations show that perovskite niobium oxide systems are not good candidates in the search for new high-${\mathit{T}}_{\mathit{c}}$ superconducting materials, but that hexagonal layered niobium oxide phases of the type considered in this paper might be of interest in studies of low ${\mathit{T}}_{\mathit{c}}$ superconducting oxides.