Investigation on the Equilibrium Phase Diagram of the Ternary System TmCl3−CdCl2−H2O at 298.15 K and Standard Molar Enthalpy of Formation of Cd3TmCl9·15H2O

Abstract

Equilibrium phase relation and solubility data for the rare earth trichloride TmCl3·6H2O and cadmium chloride in aqueous media were investigated. The corresponding equilibrium phase diagram in the ternary system TmCl3−CdCl2−H2O at 298.15 K was constructed. The results showed that the system was complicated and consisted of four stable equilibrium solid phases [CdCl2·2.5H2O, CdCl2·H2O, Cd3TmCl9·15H2O (3:1 type), TmCl3·6H2O]. The new compound Cd3TmCl9·15H2O was found to be congruently soluble in the system. The 3:1 type compound obtained was identified and characterized by the method of X-ray diffraction, X-ray diffraction single-crystal structure analysis, thermogravimetry, and differential thermogravimetry (TG, DTG). It belongs to a monoclinic system, with space group P2(1)/n, a = 1.2267(4) nm, b = 2.0034(7) nm, c = 1.2321(4) nm, β = 113.793(5)°, V = 2.7706(17) nm3, Dc = 2.554 g·cm−3, Z = 4. The standard molar enthalpy of solution of Cd3TmCl9·15H2O in deionized water was measured to be (29.73 ± 0.40) kJ·mol−1 by heat conduction microcalorimetry. Its standard molar enthalpy of formation was calculated to be −(6747.2 ± 1.2) kJ·mol−1.