Raman mapping in the elucidation of solid salt eutectic and near eutectic structures

The distribution of the different components of solidified eutectic or near-eutectic salt mixtures (eutectics) was examined by use of Raman microscope mapping of the structures formed when these melts were slowly cooled. Seven binary and one ternary system were investigated. In most cases the component crystallized phases consisted of roughly rounded areas of about 0.5-5 µm across, the areas alternating in all directions across the sections. These three-dimensional structures may best be described by the term conglomerate. The size of these areas depended on the cooling rate and the composition. When unidirectional cooling was applied it was possible for the system (KCl-Na2SO4, 60 : 40 mol/mol) to observe lamellar arrangements of the component phases, in an arrangement closely similar to what is frequently found among metallic or ceramic eutectics. Each area, conglomerate or lamellar, did not consist of a pure chemical component, although having one component in a high concentration. Probably the behaviour represents separation on solidification due to the limiting solid solubility. The conglomerate structures are very different from what has been found for metallic or ceramic eutectics. Small changes in composition, in sectioning direction and in solidification technique were found to result in relatively small differences. The major effect was found to result from the rate of solidification, faster cooling causing markedly smaller rounded areas with the conglomerate becoming much finer grained. The high area of interphasial contact between the solid solutions is considered to be responsible for the unexpectedly high electrical conductivity previously found and to give rise to part of the melting enthalpy differences between those of solidified salt eutectics and those of the corresponding unmelted mechanical mixtures of the component salts. Mixtures with a larger variation away from the eutectic composition also showed a conglomerate structure, but with bigger and more irregular areas.