Study on Phase Equilibrium in the Quaternary System CsCl−LuCl3−HCl (10.06 %)−H2O at 298.15 ± 0.1 K and New Solid-Phase Compounds

The equilibrium solubility of the quaternary system CsCl−LuCl3−HCl−H2O was determined at 298.15 K, and the corresponding equilibrium diagram was constructed. The quaternary system is complicated with five equilibrium solid phases, CsCl, Cs4LuCl7·5H2O (4:1 type), Cs9Lu5Cl24·29H2O (9:5 type), Cs2LuCl5·2H2O (2:1 type), and LuCl3·6H2O, of which the new compounds Cs4LuCl7·5H2O and Cs9Lu5Cl24·29H2O were found to be congruently soluble and the new compound Cs2LuCl5·2H2O was found to be incongruently soluble in the system. The three new compounds obtained were identified and characterized by the methods of X-ray diffraction, thermogravimetry, and differential thermogravimetry. The standard molar enthalpy of solution of Cs4LuCl7·5H2O, Cs9Lu5Cl24·29H2O, and Cs2LuCl5·2H2O in deionized water was measured to be (15.6 ± 0.3) kJ·mol-1, (−78.8 ± 0.6) kJ·mol-1, and (−62.5 ± 0.5) kJ·mol-1, respectively, by heat conduction microcalorimetry. Their standard molar enthalpies of formation were calculated to be (−4313.0 ± 2.1) kJ·mol-1, (−17871.6 ± 5.3) kJ·mol-1, and (−2526.5 ± 1.2) kJ·mol-1, respectively.