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 [1]: 1) α-Bi2O3 monoclinic phase, stable at room temperature; 2) β-Bi2O3 tetragonal phase; 3) γ-Bi2O3 cubic phase; 4) high-temperature δ-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 [2]. The main attention in literature is paid to δ-Bi2O3 phase stabilization, but complex polymorphism of Bi2O3-based compounds is also very interesting. Earlier, the ternary systems Bi2O3-Dy(Er)2O3-WO3 and Bi14W1-xLaxO24-3x/2 solid solutions were investigated with point of view of the stabilization of the oxygen-conducting δ-Bi2O3 cubic phase [3-5]. Similar Nd2MoO6-Bi2O3 and Bi2O3-Ln2O3-MoO3 (La, Pr) systems were investigated in our studies, where extensive stability fields of tetragonal and cubic compounds were found [6–8]. Full Bi2O3-Ln2O3- WO3 (Ln = La, Pr, Nd) systems with tungsten and large rare-earth cations have not been investigated. This work is devoted to the study of phase formation and polymorphism of Bi2O3-based compounds in the ternary Bi2O3-Ln2O3-WO3 systems (Ln = La, Pr, Nd) using differential scanning calorimetry and electrophysical measurements. Polycrystalline samples were obtained by solid state synthesis in the air. Low cooling rate (5 K/min) have been used to avoid metastable phases. Numerous phases with a cubic (δ and δ'), tetragonal (β and Bi14WO24-type), monoclinic (Bi3.24Ln2W0.76O10.14-type), and rhombohedral structure were observed in Bi2O3-Ln2O3-WO3 systems depending on Ln type and Bi2O3 concentration. The monoclinic Bi3.24Ln2W0.76O10.14-type compounds have the widest area of stability for all Ln (La, Pr, Nd). The high-temperature oxygen-conducting δ-Bi2O3 phase with a cubic fluorite structure stabilizes in all investigated systems in a narrow range of concentrations (85-90 mol.% Bi2O3). However, in the case of Ln = Nd, the additional stability field of cubic fluorite-type compounds (δ1-Bi2O3 phase) was found near the Nd2WO6-NdBiO3 join. DSC investigations show, that cubic samples (δ and δ' phases) do not suffer phase transformation. Tetragonal samples with β-Bi2O3 structure and monoclinic Bi3.24Ln2W0.76O10.14- type compounds demonstrate complex polymorphism with two phase transitions at heating, and one transition during cooling. Above the phase transitions these compounds are transformed into high temperature cubic fluorite-type phase. One phase transition between tetragonal and cubic phases was found for Bi14WO24-type samples both for heating and cooling. Cubic δ-Bi2O3 and tetragonal β-Bi2O3 samples demonstrate high electrical conductivity (~ 0.1-0.6 S/cm at 800 °C). The conductivity of monoclinic and cubic compounds with lower Bi2O3 concentration reaches 0.1-0.01 S/cm at 800 °C.