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

Abstract

The equilibrium solubility of the quaternary system CsCl−LuCl 3 −HCl−H 2 O was determined at 298.15 K, and the corresponding equilibrium diagram was constructed. The quaternary system is complicated with five equilibrium solid phases, CsCl, Cs 4 LuCl 7 ·5H 2 O (4:1 type), Cs 9 Lu 5 Cl 24 ·29H 2 O (9:5 type), Cs 2 LuCl 5 ·2H 2 O (2:1 type), and LuCl 3 ·6H 2 O, of which the new compounds Cs 4 LuCl 7 ·5H 2 O and Cs 9 Lu 5 Cl 24 ·29H 2 O were found to be congruently soluble and the new compound Cs 2 LuCl 5 ·2H 2 O 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 Cs 4 LuCl 7 ·5H 2 O, Cs 9 Lu 5 Cl 24 ·29H 2 O, and Cs 2 LuCl 5 ·2H 2 O 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.