Phase Equilibrium System of CsCl–TbCl3–HCl(∼7.6 mass %)–H2O at 298.2 ± 0.1 K and Fluorescent and Thermal Properties of Its Solid-Phase Compounds in the System

The solubility data of the quaternary system CsCl–TbCl3–HCl(∼7.6 mass %)–H2O at 298.2 K were determined, and the corresponding phase diagram according to central projection data on the trigonal basal face CsCl–TbCl3–H2O was plotted. The diagram includes three invariant points and four crystallization fields corresponding to CsCl, Cs5TbCl8·6H2O, Cs2TbCl5·6H2O, and TbCl3·6H2O. Two new solid-phase compounds of Cs5TbCl8·6H2O and Cs2TbCl5·6H2O, which are congruently soluble in the system, were isolated and characterized by the methods of powder X-ray diffraction, thermogravimetric, and differential thermal analysis. The standard molar enthalpies of solution for the two compounds of Cs5TbCl8·6H2O and Cs2TbCl5·6H2O were measured to be (60.64 ± 0.28) and (14.08 ± 0.45) kJ·mol–1 by heat conduction microcalorimetry under the conditions of infinite dilution, and their standard molar enthalpies of formation were calculated to be −(5087.1 ± 1.7) kJ·mol–1 and −(3764.2 ± 1.0) kJ·mol–1. The fluorescence excitation and emission spectra of Cs5TbCl8·6H2O and Cs2TbCl5·6H2O were measured. The results show that upconversion spectra exhibit for Cs5TbCl8·6H2O at 500 nm and 545 nm and for Cs2TbCl5·6H2O at 495 nm and 545 nm excited at 750 nm, and the upconversion intensity increases with the increase of TbCl3 contents in CsCl.