Oxide ion distribution, vacancy ordering and electrical behaviour in the Bi₃NbO₇–Bi₃YbO₆pseudo-binary system

Abstract

Oxide ion distribution, vacancy ordering and electrical conductivity has been examined in the Nb/Yb double substituted bismuth oxide based system Bi 3 Nb 1-x Yb x O 7-x , using X-ray and neutron powder diffraction, reverse Monte Carlo modelling of total neutron scattering data and a.c. impedance spectroscopy. Transference number measurements confirm the system to be predominantly ionically conducting above ca. 450°C. Niobium rich compositions show incommensurate ordering of the fluorite subcell, while increasing ytterbium content results in a commensurate fluorite, with fully disordered cation and anion sublattices. Oxide ion distribution shows both compositional and thermal dependencies. The latter is discussed with respect to its effect on the thermal variation of cubic lattice parameter. Substitution of bismuth by niobium and ytterbium in the cation sublattice of bismuth oxide leads to the creation of Frenkel interstitial oxide ions, which increase the tetrahedral vacancy concentration. The high vacancy concentration is confirmed in both Rietveld and RMC analyses of neutron data. Examination of vacancy ordering, in the x = 0.6 composition, indicates a favouring of <100> vacancy pair alignment.

The Fluorite-Like Phase Nd5Mo3O16±δ in the MoO3–Nd2O3 System: Synthesis, Crystal Structure, and Conducting Properties

“…The preferential short-range ordering of oxygen vacancies with anion-deficient fluorites has been determined in a number of systems using NPD methods, with ⟨111⟩ pairs formed in aliovalently doped ZrO 2 and CeO 2 − , and ⟨110⟩ or ⟨100⟩ pairs observed in doped Bi 2 O 3 compounds. − In general, the formation of such anion vacancy pairs with increasing vacancy concentration hampers ionic diffusion and leads to a reduction in the bulk ionic conductivity (see, for example, refs − ). However, anion-deficient Nd 5 Mo 3 O 16+δ (i.e.…”

An ab initio study of oxide ion dynamics in type-II Bi3NbO7

“…12,13 The type-II structure is characterized by a superlattice, which shows a complex three-dimensional incommensurate modulation, but can be satisfactorily modelled using the fcc uorite subcell. Subvalent substitution of Nb 5+ in Bi 3 NbO 7 increases the nominal vacancy concentration and ionic conductivity, as seen for example in the Bi 3 YO 6 -Bi 3 NbO 7 , 17,18 Bi 3 YbO 6 -Bi 3 NbO 7 20 and Bi 3 ZrO 6.5 -Bi 3 NbO 7 21 pseudo-binary systems. These linked niobate polyhedra are believed to be responsible for a signicant electronic contribution to total conductivity at low temperatures, perhaps through a polaron hopping mechanism.…”