Electronic structure of Aurivillius phases: Ideal Bi2NbO6, its stabilization with fluorine substitution and the role of oxygen vacancies

The electronic structure of the first member of the Aurivillius family of phases Bi2O2(Mm-1NbO3m+1) with m=1 has been calculated using the linear muffin-tin orbital (LMTO) method. The band structure of Bi2NbO6 is similar to that of Bi2O3, which is the best oxygen ionic conductor. The Fermi level is located at the strong O 2p peak and reveals the formation of a large number of oxygen holes in the valence band. The substitution of fluorine for oxygen is shown to lead to a stabilization of the structure and the formation of Bi2NbO5F. The preferred sites for fluorine substitution have been determined based on substitution energy estimates. The role of oxygen vacancies on the electronic structure and stability of the phases is investigated. Ideal Bi2NbO6 is shown to be unstable and tends to contain a large number of oxygen vacancies. Vacancy formation energies have been found for different oxygen sites of the ideal crystals.