Constitution and Microstructure of Some Binary Alkali Halide Mixtures

The constitutions of binary alkali halide systems are considered for the fluorides, chlorides, and bromides of lithium, sodium, and potassium. These systems are divided equally into those that are simple binary systems having a common ion and those that are capable of double decomposition into reciprocal salt pairs, AB + CD ⇌ AD + BC. The simple binary systems having been examined previously, the present work was principally concerned with the constitution of the pseudobinary systems. Using thermal analysis, X-ray methods, and microscopy, the liquidus and temperature horizontals were determined for 18 such systems and the equilibria were interpreted with reference to quaternary systems. The tendency of a given salt pair to precipitate the reciprocal salts may be related to the relevant lattice energies. Microscopic examination of directionally frozen specimens revealed that true binary eutectic mixtures freeze with regular two-phase distributions very similar to those observed in metallic eutectics; these distributions are illustrated.