Bi2O3–Nd2O3–WO3 system: Phase formation, polymorphism, and conductivity

Abstract

Cubic, tetragonal, and monoclinic (Bi2O3)x (Nd2O3)y (WO3)z (x + y + z = 1) solid solutions based on the Bi2O3 oxygen ion conductor have been prepared by solid-state reactions in the ternary system Bi2O3–Nd2O3–WO3. The field of monoclinic compounds with a Bi3·24La2W0·76O10.14-type structure has been shown to account for most of the ternary system. Compounds with a cubic fluorite structure exist at the boundary of the monoclinic phase field in two small regions at high (83–91 mol% Bi2O3, δ-phase) and low (20–55 mol% Bi2O3, δ′-phase) Bi concentrations. The cubic samples of the δ-phase retain their structure only during rapid heating and cooling, but annealing in the range of 300–700 °C results in structure degradation to lower symmetry phases. The monoclinic compounds and Bi-poor cubic compounds (δ′-phase) have good thermal stability. The cubic samples of the δ′-phase are hygroscopic. Their bulk conductivity noticeably increases with atmospheric humidity, suggesting that these materials are potential proton conductors.