Phase Equilibria in the CsBr–TbBr3–H2O and CsBr–TbBr3–HBr (∼13.0 wt %)–H2O Systems at 298.1 K and the Standard Molar Enthalpy of Formation for Cs3TbBr6·10H2O

The phase equilibria of the ternary system CsBr–TbBr3–H2O and quaternary system CsBr–TbBr3–HBr (∼13.0 wt %)–H2O were studied by using the method of isothermal dissolution equilibrium at 298.1 K. The phase diagrams of the above two systems were constructed by using the experimental data. The results show that the ternary system CsBr–TbBr3–H2O consists of three crystallization regions (CsBr, Cs3TbBr6·10H2O, and TbBr3·6H2O). The phase diagram in the quaternary system CsBr–TbBr3–HBr (∼13.0 wt %)–H2O is a type of limit solid solution, and there are three crystallization fields: CsBr, solid solution CsxTbBr(3+x)·6H2O (x = 1.25–7.00), and TbBr3·6H2O. Cs3TbBr6·10H2O is incongruently soluble in water. The experimental solubility data of terbium bromide in water at 298.1 K is 65.42 mass %. The compound Cs3TbBr6·10H2O was characterized by X-ray diffraction and thermogravimetry–derivative thermogravimetry techniques. The standard molar enthalpy of a solution of (48.25 ± 0.61) kJ/mol in water for Cs3TbBr6·10H2O was measured using microcalorimetry at 298.15 K, and its standard molar enthalpy of formation was determined as being −(5093.4 ± 1.8) kJ/mol.