Nucleation Studies on Ionic Melts

INTRODUCTION Previous work on crystal nucleation in one-component melts has largely been confined to molecular liquids or metals. Molecular liquids have been studied in bulk", and in the form of clouds of droplets produced by condensation in supersaturated vapourv". Metals have also been studied in bulk 7, 8, and as droplets of microscopic size. These were produced either singly by melting small fragments 9, 10, or in large numbers in emulsions-l12. For some purposes it is an advantage, in studying supercooling, to subdivide a sample of melt into a large number of droplets, thereby reducing to a small fraction the number which might contain impurities. In particular, only droplets free from impurities may be relied upon to exhibit phenomena of spontaneous nucleation. Apart from work on glasses, where crystallization is very slow, little information exists on the spontaneous nucleation of ionic melts. In spite of experimental difficulties due to their high melting points, work on inorganic salts containing simple ions seems likely to prove rewarding, especially in view of the additive properties ofthe inter-ionic forces. Central, Coulombic force fields are more amenable to theoretical treatment than those in other classes of compounds, as is evident from the considerable success with which they have been used to predict the lattice energies of ionic solids. A short description is given below of a new technique for studying the solidification of molten salts at high temperatures, together with some preliminary results obtained with alkali halides.