Investigation of Phase Equilibria on LiCl-KCl-GdCl3 Ternary System

Abstract

The LiCl-KCl-GdCl₃ quasi-ternary phase diagram has been investigated by differential thermal analysis (DTA) and x-ray diffraction (XRD) techniques. These experiments showed that the LiCl-KCl-GdCl₃ system consists of several pseudo-ternary and quasi-ternary composition triangles. Coexisting phases in the solid state at 260 °C were characterised by XRD analysis of the equilibrated samples. Quasi-ternary compounds were not found. The ternary eutectic temperature was observed at 315 ± 2 °C and its composition was delineated at 57 mol.% LiCl-42 mol.% KCl-1 mol.% GdCl₃ in the LiCl-KCl-K₃GdCl₆ composition triangle. The other ternary eutectic temperature was observed at 308 ± 2 °C in the LiCl-K₃GdCl₆-K₂GdCl₅ composition triangle. Its composition was deduced at 40.2 mol.% LiCl-41.7 mol.% KCl18.1 mol.% GdCl₃. Another ternary eutectic temperature was observed at 360 ± 9 °C in the LiCl-K₂GdCl₅-KGd₂Cl₇ composition triangle. Its composition was deduced at 33.8 mol.% LiCl-29.5 mol.% KCl-36.7 mol.% GdCl₃. The fourth ternary eutectic temperature was observed at 362 ± 4 °C in the LiCl-KGd₂Cl₇-LiGdCl₄ composition triangle. Its composition was delineated at 48 mol.% LiCl-11 mol.% KCl-41 mol.% GdCl₃. The ternary pseudo-peritectic reaction “L+GdCl₃ ↔ LiGdCl₄+KGd₂Cl₇” was observed at 370 ± 3 °C. Its composition was delineated at 46.7 mol.% LiCl-10.5 mol.% KCl-42.8 mol.% GdCl₃. The primary and secondary crystallisation temperatures, as well as polymorphic phase transformation temperatures of the samples were deduced from the heating runs of DTA experiments. Further, phases responsible for the various thermal events were ascertained. Isothermal composition triangles at selected temperatures, Scheil diagram and the polythermal liquidus projection with isothermal contours of the ternary phase field were drawn.