Phase Diagram of the Ternary System NaCl/CsCl/H₂O Elucidated by Mechanochemical Equilibration

Mechanochemical equilibration of the ternary system NaCl/CsCl/H₂O was investigated using two types of grinders: (I) a ball mill (a closed system) and (II) a mortar grinder (an open system). Products obtained by grinding mixtures with different molar ratios of NaCl, CsCl, and H₂O ware determined by X-ray diffraction, IR, NMR, and thermal analysis. These are either solid solutions of hydrated NaCl in CsCl, Cs_1-x[Na(H₂O)]_xCl, or a hydrated double salt, Cs[Na(H₂O)]₂Cl₃. By applying the ball mill grinder, it was possible to construct a phase diagram of the NaCl/CsCl/H₂O system in the area in which only solids exist in the equilibrium state. In the mortar grinder the ground material was in contact with the atmosphere and water was exchanged between the solids and the atmosphere (adsorption or desorption). Solid solutions were obtained by both techniques whereas the double salt was obtained only by ball mill grinding. The differences in the results of the two grinding techniques were explained on the basis of the phase diagram.