Solid-liquid equilibria for the systems of (CsCl + DyCl3 + H2O) and {CsCl + DyCl3 + HCl (∼10.7%) + H2O} at T = 298.15 K and the thermodynamic properties of the new solid compounds

Abstract

Solid–liquid equilibria in the ternary system (CsCl + DyCl3 + H2O) and the quaternary system {(CsCl + DyCl3 + HCl (∼10.7%) + H2O} at T = 298.15 K and under 101.3 kPa were investigated by the method of isothermal solution saturation, and the corresponding phase diagrams were constructed. It was found that there were three crystallization regions corresponding to CsCl, Cs2DyCl5·6H2O and DyCl3·6H2O in the ternary system. Nevertheless, four crystallization regions corresponding to CsCl, Cs5DyCl8·6H2O, Cs2DyCl5·6H2O and DyCl3·6H2O were found in the quaternary system. The compound Cs2DyCl5·6H2O was congruently soluble in the two systems, and new double salt Cs5DyCl8·6H2O was formed in the quaternary system which was congruently soluble in an average medium of ∼10.7 mass% HCl. The new solid-phase compounds Cs5DyCl8·6H2O and Cs2DyCl5·6H2O were characterized by chemical analysis, XRD and TG-DTG techniques. The standard molar enthalpies of solution of Cs5DyCl8·6H2O and Cs2DyCl5·6H2O in water were measured to be (44.28 ± 0.50) kJ·mol−1 and (5.32 ± 0.27) kJ·mol−1 by microcalorimetry in the condition of infinite dilution, and their standard molar enthalpies of formation were determined to be –(5086.9 ± 1.3) kJ·mol−1 and –(3771.6 ± 0.9) kJ·mol−1. As part of this work, five compounds Cs3LaCl6·3H2O, CsLaCl4·4H2O, CsCeCl4·4H2O, CsPrCl4·6H2O and CsNdCl4·9H2O were synthesized according to the literature [18–21]. The molar enthalpies of solution of Cs3LaCl6·3H2O, CsLaCl4·4H2O, CsCeCl4·4H2O, CsPrCl4·6H2O and CsNdCl4·9H2O in water were also measured to be (14.78 ± 0.31) kJ·mol−1, –(30.98 ± 0.62) kJ·mol−1, –(20.26 ± 0.22) kJ·mol−1, –(19.64 ± 0.54) kJ·mol−1 and –(5.86 ± 0.45) kJ·mol−1 in the condition of infinite dilution, and their standard molar enthalpies of formation were calculated to be –(3351.1 ± 1.1) kJ·mol−1, –(2740.3 ± 1.0) kJ·mol−1, –(2746.1 ± 0.7) kJ·mol−1, –(3326.8 ± 0.9) kJ·mol−1 and –(4189.7 ± 0.9) kJ·mol−1, respectively.