Bi_1-xNb_xO_1.5+x (x=0.0625, 0.12) fast ion conductors: Structures, stability and oxide ion migration pathways

A combined experimental and computational study of Bi_1-xNb_xO_1.5+x (x=0.0625 and 0.12) has been carried out using laboratory X-ray, neutron and electron diffraction, impedance measurements and ab-initio molecular dynamics. We demonstrate that Bi_0.9375Nb_0.0625O_1.5625, previously reported to adopt a cubic fluorite-type superstructure, can form two different polymorphs depending on the synthetic method: a metastable cubic phase is produced by quenching; while slower cooling yields a stable material with a tetragonal √2×√2×1 superstructure, which undergoes a reversible phase transition into the cubic form at ~680 °C on subsequent reheating. Neutron diffraction reveals that the tetragonal superstructure arises mainly from ordering in the oxygen sublattice, with Bi and Nb remaining disordered, although structured diffuse scattering observed in the electron diffraction patterns suggests a degree of short-range ordering. Both materials are oxide ion conductors. On thermal cycling, Bi_0.88Nb_0.12O_1.62 exhibits a decrease in conductivity of approximately an order of magnitude due to partial transformation into the tetragonal phase, but still exhibits conductivity comparable to yttria-stabilised zirconia (YSZ). Ab-initio molecular dynamics simulations performed on Bi_0.9375Nb_0.0625O_1.5625 show that oxide ion diffusion occurs by O^2- jumps between edge- and corner-sharing OM₄ groups (M=Bi, Nb) via tetrahedral □M₄ and octahedral □M₆ vacancies.