Effect of Crystallographic Anisotrophy and Lanthanum Stoichiometry on Microstructural Evolution of Lanthanum Silicate Electrolytes

Abstract

Lanthanum silicates with different lanthanum concentrations La9.33+x(SiO4)6O2+1.5x were prepared by solid state reaction using milling procedures of different degrees and sintered at 1700 degrees C or 1750 degrees C. The phase composition and the microstructural evolution were shown to depend strongly on the heat-treatment temperature and history. The presence of the liquid phase in the lanthanum-rich compositions at sintering temperatures is clearly evidenced, which, however, is not in accordance with the known phase relations of the material. Exaggerated growth of plate-shaped grains in the c-axis direction was induced by the inhomogeneous presence of the liquid phase. The solid solution region with excess lanthanum appears to extend with increasing temperature, as evidenced by intragranular second phase precipitation on heat treatments at lower temperature as well as the X-ray powder diffraction results of differently treated samples. Lanthanum-deficient compositions were shown to exist as a two-phase mixture with La2Si2O7 in equilibrium, although the powders prepared by solid state reaction at 1300 degrees C do not indicate the significant presence of a secondary phase, regardless of the composition. The groove profiles with both a maximum and minimum (or hump) indicate the surface diffusion mechanism controlled by bulk defect chemistry and defect migration in the material. This phenomenon can be explained by the transport anisotropy in La diffusion which controls the matter transport by the bulk ambipolar diffusion mechanism, supported by recent theoretical as well as experimental studies. Electron backscattered diffraction mapping clearly correlated the grains perpendicular to the c-axis with the strong development of thermal grooving and vice versa by the transport anisotropy. There is little indication of the surface and grain-boundary energy anisotropy regardless of the compositions.