The melting enthalpies of three alkali metal nitrate/nitrite eutectics (NaN03-KN03, LiN03-KNO3 and NaN02-NaN03-KN03) show statistically significant differences between the powdered solids (i.e. premelted, solidified and ground) and their mechanical mixtures (i.e. unmelted) to a greater extent than can be accounted for by the mixing enthalpies of their components. Preliminary results on another ternary eutectic (LiN03-NaN03-KN03) show similar differences.
This effect is considered to arise from the structures of the eutectics, where there is alternation of very small volumes of the different phases, and hence a high proportion of ions in interfacial layers where the ionic sites are of higher energy. SEM has shown structure, perhaps one nanometre in width, on broken surfaces of premelted NaN02-NaN03-KN03 eutectic.