We have determined the thermal characteristics and optical properties of the sulfide and oxysulfide phases in the Nd₂S₃ - Nd₂O₃ system.

A congruent melting peak at temperature 1801 ± 4.9 °C with ΔH = 65.2 ± 6.7 kJ/mol was detected for the Nd₂S₃ compound by the DSC method. The characteristics of the α-Nd₂S₃ → γ-Nd₂S₃ polymorphic transition are t = 1183 ± 1.8°С, and ΔH = 7.5 ± 0.3 kJ/mol. The γ-Nd₂S₃ phase obtained upon cooling during annealing at 800 °C is retained for up to 30 h, and then the γ-Nd₂S₃ → α-Nd₂S₃ transition occurs within 20 h. The microhardness of the phases is: α-Nd₂S₃ H = 451 ± 4 HV; γ-Nd₂S₃ H = 531 ± 4 HV.

It was found by the TG method that the Nd₁₀S₁₄O phase thermally dissociates at temperatures above 1400 °C. The mass loss is 0.5 mass % at 1580 °C and 1.0 mass % at 1620 °C, but the samples remain single-phase ones after cooling. However, two impurity phases γ-Nd₂S_3-X and Nd₂O₂S appear in the Nd₁₀S₁₄O samples treated at temperatures above 1620 ± 20 °C.

For samples of the Nd₁₀S₁₄O phase annealed in an argon atmosphere at temperatures of 1050, 1400, 1580 °C, a regular decrease in the unit cell parameters and optical band gap was recorded: 1050 °C a = 15.06291(28), c = 19.97864(35), E_g = 2, 63 eV, 1400 °C a = 15.04779(36), c = 19.97160(44), E_g = 2.64 eV; 1580 °C a = 15.03532(48), c = 19.94984(60), E_g = 2.51 eV. The microhardness of Nd₁₀S₁₄O is H = 549 ± 10 HV. The Nd₂O₂S phase has H = 593 ± 4 HV, E_g = 4.28 eV.

The phase diagram of the Nd2S3 - Nd2O3 system from 1000 °C to the melt was constructed. The Nd2O2S phase melts congruently at 2050 ± 30 °C. Eutectics with coordinates 23 mol. % Nd2O3 (0.3484 Nd10S14O + 0.6516 Nd2O2S), t = 1553 ± 1.8°С; ΔH = 187 ± 19 J/g; 82 mol. % Nd2O3; (0.54 Nd2O2S + 0.46 Nd2O3), t = 1970 ± 30°С were obtained. The liquidus of the Nd2S3 - Nd2O3 system was built according to DSC data and calculated using the Redlich-Kister equation. The melting enthalpy of Nd₂O₂S ΔH = 67 kJ/mol was calculated using the Schroeder equation.