abstract
Starting from a previously published stoichiometric model for the commensurate Type III phase in the (1 - x)Bi2O3 center dot xNb(2)O(5) system, Bi94Nb32O221 (x = 0.254), we have developed a crystal-chemical model of this phase across its solidsolution range 0.20 <= x <= 0.26. After using annular dark-field scanning transmission electron microscopy to identify the metal sites that support nonstoichiometry, we show that the maximum possible range of that nonstoichiometry is 0.198 <= x <= 0.262, perfectly consistent with the experimental result. Intersite cation defects on these sites provide some local coordinative flexibility with respect to the surrounding oxygen sublattice, but not enough to create continuous fluorite-like channels like those found in the high-temperature incommensurate Type II phase. This explains the reduced oxide-ionic conductivity of Type III compared to Type II at all temperatures and compositions, regardless of which phase is thermodynamically stable under those conditions. The solid-solution model shows that oxygen disorder and vacancies are both reduced as x increases, which also explains why Type III becomes relatively more stable, and why oxide ionic conductivity decreases, as x increases.
keywords
BI2O3-NB2O5 SOLID-SOLUTION; BI2O3-TA2O5 SYSTEM; FLUORITE; BI3NBO7; PHASE; TEMPERATURE; MODULATION; CHEMISTRY; BI2O3-WO3
subject category
Chemistry; Materials Science
authors
Wind, J; Sharma, N; Yaremchenko, AA; Kharton, VV; Blom, DA; Vogt, T; Ling, CD
our authors
Aleksey Yaremchenko
Principal Researcher
Vladislav Kharton
Invited Principal ResearcherProjects
acknowledgements
This work was supported by the Australian Research Council (Discovery Project scheme). J.W. acknowledges financial support from the Australian Institute of Nuclear Science and Engineering (Postgraduate Research Awards scheme). V.V.K. acknowledges financial support from the Russian Science Foundation (project 17-79-30071) and Russian Ministry of Education and Science (project 14.B25.31.0018). A.Y. acknowledges financial support from the FCT, Portugal (projects IF/01072/2013/CP1162/CT0001 and CICECO-Aveiro Institute of Materials POCI-01-0145-FEDER-007679 (FCT ref UID/CTM/50011/2013)). D.B. and T.V. thank the VPR of the University of South Carolina for support operating the JEOL 2100F.