Thermal Diffusion and Related Phenomena in Cubic (fcc and bcc) Sulfate Lattices

Abstract

Thermal diffusion of cations has been studied in face-centered cubic Li2SO4 containing small amounts of other sulfates (temp. range 590—750°C), in body-centered cubic LiAgSO4 containing about 0.5% of other alkali sulfates (450—550°C) and in body-centered cubic LiNaSO4 (546 to 588°C). For the fcc systems the size of the added cation was of great importance for the thermal diffusion. Thus, for the small Na+ and Ag+ ions, the Soret-coefficients (σ) are of the order of only 10-4 degr.-1. The direction has not been established with certatinty, although there are indications that Ag+, and also Ca2+, are enriched on the cold side, while Na+ might be slightly enriched on the hot side. There is a significant enrichment of the large alkali ions (K+, Rb+, Cs+) on the hot side; σ=— 2 × 10-3 degr.-1 for K+. In some experiments with these large alkali ions phase boundaries were intersected, and the large ion was always enriched in the high-temperature phase. In bcc Li2SO4—Ag2SO4 (containing 40 mole % Li2SO4) the light component Li+ was enriched at the hot side: σ ≦ 1.7 × 10-3 degr.-1. Regarding added alkali ions, Na+ is enriched at the hot side (σ ∼ 2 × 10-3 degr.-1), while the direction is not established with certainty for K+ and Rb+. In bcc LiNaSO4 no significant separation of the two cations could be detected, and the Soret-coeffi-cient is certainly less than 10-3 degr.-1. A comparison of fcc and bcc sulfates with molten salts regarding thermal diffusion, electromigration mobility of cations, and the conductivity changes caused by impurities, shows a distinction between the fcc systems on one side and bcc sulfates and melts on the other. This supports previous conclusions that the fcc systems are characterised by a higher degree of order than the bcc ones.