ИСТИНА

Bi2O3-based solid solutions are interesting as promising oxygen conductors with conductivity reaching 0.1-1 S/cm at 800 °C. The pure Bi2O3 has a complex polymorphism. Four basic Bi2O3 phases are well known [Kharton et al., 2001]: 1) -Bi2O3 monoclinic phase, stable at room temperature; 2) -Bi2O3 tetragonal phase; 3) -Bi2O3 cubic phase; 4) high-temperature -Bi2O3 cubic phase with fluorite structure, which is stable in narrow temperature range 700-780 ºC and demonstrate extremely high oxygen conductivity near 3 S/cm [Takahashi et al., 2001]. The main attention in literature is paid to the stabilization of the oxygen-conducting -Bi2O3 cubic phase at room temperature by doping. The best results were achieved with two different codopants. From this point of view, the ternary systems Bi2O3-Dy(Er)2O3-WO3 [Jiang et al., 2002; Watanabe et al., 2005] and Bi14W1-xLaxO24-3x/2 solid solutions [Borowska-Centkowska et al., 2011] were investigated. In addition to the -Bi2O3 cubic phase, as a result of doping, less symmetric Bi2O3-based phases can arise, but little attention is paid to their phase formation and polymorphism. In this work, we investigated the phase formation, polymorphism and conductivity of Bi2O3-based compounds with different symmetry in ternary Bi2O3-Ln2O3-MeO3 systems (Ln = La, Pr, Nd, Me = W, Mo). Polycrystalline samples were obtained by solid state synthesis in air. Different phases with a cubic, tetragonal, monoclinic, and rhombohedral structure were observed in Bi2O3-Ln2O3-MeO3 systems as the composition was changed. In the systems with Me = Mo, phases with a cubic and tetragonal structure prevail. In systems with Me = W, the fields of monoclinic and rhombohedral phases are most pronounced, while cubic and tetragonal compounds form in very narrow regions of compositions. So, for Me = W,the high-temperature cubic -Bi2O3 phase is stabilized in a range of 85-90 mol.% Bi2O3, which is much lower than in similar systems with Me = Mo (55-90 mol.% Bi2O3). When Ln = Pr, Nd, two fields of cubic compounds with a fluorite structure are formed in Bi2O3-Ln2O3-MeO3 ternary systems in regions with high and low bismuth concentrations. The studies have shown that the concentration of bismuth oxide is the main factor affecting on the magnitude of oxygen conductivity, but for all Bi2O3-based samples the conductivity remains quite high. At low bismuth concentration (45-50 mol% Bi2O3) the conductivity of the samples is close to 0.01 S/cm at 800 °C. At high bismuth concentration (85-90 mol.% Bi2O3) cubic -Bi2O3 samples demonstrate conductivity, reaching 0.6 S/cm at 800 °C. For most samples, the conductivity obeys the Arrhenius law with activation energy near 0.9-1.2 eV. For cubic samples with high bismuth concentration, 85-90 mol.%, Bi2O3, above 400 °C, the conductivity can be approximated by the Vogel-Fulcher-Tamman law. Kharton V.V., Naumovich E.N., Yaremchenko A.A., Marques F.M.B. J. Solid State Electrochem., 2001, 5, 160-187. Takahashi T., Iwahara H., Nagai Y. J. Appl. Electrochem., 1972, 2, 97-104. Jiang N., Wachsman E.D., Jung S.H. Solid State Ionics, 2002, 150, 347-353. Watanabe A., Sekita M. Solid State Ionics, 2005, 176, 2429-2433. Borowska-Centkowska A., Krok F., Abrahams I., Wrobel W. Solid State Ionics, 2011, 203, 22-28.