Local Structure Adaptations and Oxide Ionic Conductivity in the Type III Stability Region of (1-x)Bi2O3 center dot xNb(2)O(5)

authors	Wind, J; Sharma, N; Yaremchenko, AA; Kharton, VV; Blom, DA; Vogt, T; Ling, CD
nationality	International
journal	CHEMISTRY OF MATERIALS
keywords	BI2O3-NB2O5 SOLID-SOLUTION; BI2O3-TA2O5 SYSTEM; FLUORITE; BI3NBO7; PHASE; TEMPERATURE; MODULATION; CHEMISTRY; BI2O3-WO3
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.
publisher	AMER CHEMICAL SOC
issn	0897-4756
year published	2018
volume	30
issue	10
beginning page	3387
ending page	3394
digital object identifier (doi)	10.1021/acs.chemmater.8b00846
web of science category	Chemistry, Physical; Materials Science, Multidisciplinary
subject category	Chemistry; Materials Science
unique article identifier	WOS:000433403800026