Thermal Diffusion and Related Phenomena in Cubic (fee and bee) Sulfate Lattices

Thermal diffusion of cations has been studied in face-centered cubic Li2S04 containing small amounts of other sulfates (temp, range 590 — 750 °C), in body-centered cubic LiAgS04 containing about 0.5% of other alkali sulfates (450—550 °C) and in body-centered cubic LiNaS04 (546 to 588 °C). For the fee 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 (a) 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; o=— 2 x 1 0 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 Li2S04—Ag2S04 (containing 40 mole % Li2S04) the light component Li+ was enriched at the hot side: ö ^ 1.7 x 10 3 degr.-1. Regarding added alkali ions, Na+ is enriched at the hot side (o ~ 2x 10 3 degr. -1), while the direction is not established with certainty for K+ and Rb+. In bcc LiNaS04 no significant separation of the two cations could be detected, and the Soret-coefficient is certainly less than 10—3 degr.-1. A comparison of fee 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 fee systems on one side and bcc sulfates and melts on the other. This supports previous conclusions that the fee systems are characterised by a higher degree of order than the bcc ones.