resumo
Ba4Nb2O9 is shown to have two basic polymorphs: a high-temperature gamma phase, which represents an entirely new structure typed and a low-temperature (x phase, which has the rare Sr4Ru2O9 structure type. The phases are separated by a reconstructive phase transition at similar to 1370 K, the kinetics of which are sufficiently slow that the gamma phase can easily be quenched to room temperature. Below similar to 950 K, both (alpha and gamma phases absorb significant amounts of water. In the case of the gamma phase, protons from absorbed water occupy ordered positions in the Structure, giving rise to a stoichiometric phase gamma-III-Ba4Nb2O9.1/3H(2)O at room temperature. gamma-III-Ba4Nb2O9-1/3H(2)O partially dehydrates, at similar to 760 K to give another stoichiometric phase gamma-II-Ba4Nb2O9.1/3H(2)O, which completely dehydrates at similar to 950 K to gamma-I- Ba4Nb2O9. The hydrated gamma phases exhibit faster protonic and oxide ionic transport than the hydrated (x phases because of the presence in the gamma phases of 2D layers containing Nb5+ cations with unusually low oxygen coordination numbers (4 or 5) separated by discrete OH groups. Hydration appears to play an important role in stabilizing the gamma phases at low temperatures, with the gamma -> alpha transition oil reheating a quenched sample occurring at higher temperatures in humid atmospheres.
palavras-chave
TEMPERATURE PROTON CONDUCTIVITY; BARIUM NIOBATE; CERAMICS; PEROVSKITES; ELECTROLYTE; CONDUCTORS; TRANSPORT; NUMBERS; OXIDES; CELLS
categoria
Chemistry; Materials Science
autores
Ling, CD; Avdeev, M; Kutteh, R; Kharton, VV; Yaremchenko, AA; Fialkova, S; Sharma, N; Macquart, RB; Hoelzel, M; Gutmann, M
nossos autores
Aleksey Yaremchenko
Investigador Principal
Vladislav Kharton
Investigador Principal Covidadoagradecimentos
This work was supported by the Australian Research Council-Discovery Projects (DP0666465), the Australian Institute of Nuclear Science and En;Engineering (AINSE) Postgraduate Research Awards scheme, the FCT. Portugal (Project PTDC/CTM/64357/2006), and the NSF, USA (Project DMR-0502765). Travel to the FRM-II research reactor for NPD data collection was supported by AINSE.