Impact of Ln cation on the oxygen ion conductivity of Ln14W4O33 (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb) tungstates

Tungstates Ln14W4O33 (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb) with a pseudorhombohedral structure have been synthesized by a short high temperature annealing (1450-1600 degrees C) from mechanically activated mixtures of oxides. The resulting ceramics were studied by X-ray diffraction, Raman and SEM analyses. The conductivity of ceramics was measured by the 4-probe direct current method in dry and wet atmospheres (oxidizing and reducing) and by impedance spectroscopy. In order to estimate the ionic and electronic contributions to the total conductivity, the dependences of the conductivity on the oxygen partial pressure (pO2) were determined at temperatures between 700 and 900 degrees C. Ln14W4O33 (Ln = Nd, Sm, Gd) tungstates have a wide electrolytic range (10- 15-1 atm) above 700 degrees C. It should be noted that under oxidizing conditions the contribution of hole con-ductivity is absent in these compounds. Among the pseudorhombohedral phases of Ln14W4O33 (Ln = Nd, Sm, Gd), Nd14W4O33 exhibits the highest oxygen-ion conductivity (4 x 10-4 S/cm at 700 degrees C). The electrolytic domain of the second half of the REE Ln14W4O33 (Ln = Dy, Ho, Er, Tm, Yb) series becomes narrower (10- 10-10- 5 atm) and at the same time has a slight slope indicating the presence of impurity electronic charge carriers. In-vestigations in a reducing atmosphere confirmed that the reduction tendency increases with decreasing lanthanide ionic radius in the Ln14W4O33 tungsten series. The variations of the total conductivity in the series of pseudorhombohedral phases Ln14W4O33 (Ln = Dy, Ho, Er, Tm, Yb) are associated with local changes in the short -range order structure (Raman spectroscopy). When Ln14W4O33 (Ln = Nd, Gd) is exposed to water at room temperature for an extended period of time, the total conductivity decreases due to the diffusion of the Nd and Gd cations into the water.