Temperatures and enthalpies of melting of Ln₂S₃ (Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) compounds

Abstract

Samples of Ln₂S₃ (Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) polymorphs were prepared from Ln₂O₃ in a CS₂ + H₂S flow at 1050–1150 °C. DSC melting traces were recorded for Ln₂S₃ compounds of yttrium lanthanides for the first time. Samples for DSC were prepared by melting and then recrystallizing polycrystalline samples in crucibles of the DSC setup. The melting peak shapes and melting ranges signified the congruent melting of Ln₂S₃ compounds. The temperatures and enthalpies of melting were determined for the prepared Ln₂S₃ polymorphs: γ-Gd₂S₃ (t _m = 1794 °C; ΔH = 56.3 kJ mol⁻¹), γ-Tb₂S₃ (t _m = 1753 °C; ΔH = 55.8 kJ mol⁻¹), γ-Dy₂S₃ (t _m = 1698 °C; ΔH = 57 kJ mol⁻¹), δ-Ho₂S₃ (t _m = 1697 °C; ΔH = 47.7 kJ mol⁻¹), δ-Er₂S₃ (t _m = 1662 °C; ΔH = 42.2 kJ mol⁻¹), δ-Tm₂S₃ (t _m = 1665 °C; ΔH = 46.9 kJ mol⁻¹), and ε-Lu₂S₃ (t _m = 1755 °C; ΔH = 53.6 kJ mol⁻¹). The Ln₂S₃ samples experienced thermal dissociation near their melting temperatures; the relevant mass losses were 0.1–0.24 mass%. After DSC, the samples remained single phases; their concentration changed to Gd₂S_2.98, Tb₂S_2.99, Dy₂S_2.98, Ho₂S_2.97, Er₂S_2.97, and Lu₂S_2.98. A δ-Tm₂S₃ sample experienced a greater mass loss (0.77 mass%) at 1500–1670 °C to acquire the concentration Tm₂S_2.91. ε-Yb₂S₃ decomposed to YbS at temperatures above 1500 °C. The melting temperatures show an inner periodicity, and the enthalpies of melting of Ln₂S₃ show the tetrad effect as a function of rLn⁺³.