Phase Equilibrium of (Na+, Cs+//Cl––H2O) System at 313.15 K: Focus on [Cs1– x (Na·H2O) x ]Cl Solid Solution

Abstract The isothermal dissolution equilibrium method was employed to investigate the phase equilibrium and phase diagram of the ternary system (Na+, Cs+//Cl––H2O) at 313.15 K. This ternary system features the coexistence of double salts and solid solutions. The solid solution, formulated as ([Cs1–x(Na·H2O)x]Cl), was characterized by wet-residue analysis, X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and thermogravimetry–differential scanning calorimetry (TG–DSC). Three representative equilibrium compositions of the solid solution and their respective proportions within the crystallization region were determined, and the results were compared with those of the two pure salts (NaCl and CsCl). The physicochemical property diagrams indicate that both density and refractive index increase with rising cesium chloride content. A comparative phase diagram of the ternary system (Na+, Cs+//Cl––H2O) was constructed over the temperature range from 298.15 to 318.15 K. The results show that as temperature increases, the crystallization region of NaCl expands, whereas the crystallization regions of CsCl·2NaCl·2H2O and [Cs1–x(Na·H2O)x]Cl simultaneously contract.