Phase equilibria of the systems of CsCl + HoCl3 + H2O and CsCl + HoCl3 + HCl (∼10%) + H2O at T = 298.15 K and the thermodynamic and fluorescent properties of the new solid-phase compounds

• Solubility was measured for CsCl + HoCl3 + H2O and CsCl + HoCl3 + HCl (∼10%) + H2O systems at T = 298.15 K.
• The phase diagrams were plotted.
• Two new solid phase compounds were characterized.
• ΔsolHm⊖ for Cs5HoCl8 · 4H2O and Cs2HoCl5 · 6H2O were determined.

(Solid + liquid) equilibria in the ternary system CsCl + HoCl3 + H2O and in the quaternary system CsCl + HoCl3 + HCl (∼10%) + H2O at T = 298.15 K and atmospheric pressure were investigated by using isothermal solution saturation and moist residue method. Based on the measured solubility data, the corresponding phase diagrams were plotted. In the ternary system, three crystallization regions corresponding to CsCl, Cs2HoCl5 · 6H2O and HoCl3 · 6H2O were found. However, there were four crystallization regions corresponding to CsCl, Cs5HoCl8 · 4H2O, Cs2HoCl5 · 6H2O and HoCl3 · 6H2O in the quaternary system. The phase diagrams of the ternary and quaternary systems were compared, and it showed that (1) new solid-phase compound Cs2HoCl5 · 6H2O was obtained which was congruently soluble in the two systems; (2) a new double salt Cs5HoCl8 · 4H2O was formed in the quaternary system which was congruently soluble in an average medium of ∼10 mass% HCl, and (3) the area of the crystallization region of CsCl decreased with the increasing concentration of HCl in the equilibrium liquid phase. The new solid-phase compounds Cs5HoCl8 · 4H2O and Cs2HoCl5 · 6H2O were characterized by chemical analysis, XRD and TG-DTG techniques. The standard molar enthalpies of solution of Cs5HoCl8 · 4H2O and Cs2HoCl5 · 6H2O in water were confirmed to be (53.09 ± 0.53) kJ · mol−1 and (4.26 ± 0.24) kJ · mol−1 by microcalorimetry in the condition of infinite dilution and their standard molar enthalpies of formation were determined to be −(4530.1 ± 1.4) kJ · mol−1 and −(3776.6 ± 0.9) kJ · mol−1. The fluorescence excitation and emission spectra of the new solid phase compounds were measured. The results indicate that upconversion spectra of the compounds Cs5HoCl8 · 4H2O and Cs2HoCl5 · 6H2O at the same wavelength 474 nm, when the former was excited at 701 nm and the latter was excited at 713 nm.

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