Properties of oxysulfide phases and phase diagram of the Nd2S3-Nd2O3 system

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 degrees C with Delta H = 65.2 +/- 6.7 kJ/mol was detected for the Nd2S3 compound by the DSC method. The characteristics of the alpha-Nd2S3 -> gamma-Nd2S3 polymorphic transition are t = 1183 +/- 1.8 degrees C, and Delta H = 7.5 +/- 0.3 kJ/mol. The gamma-Nd2S3 phase obtained upon cooling during annealing at 800 degrees C is retained for up to 30 h, and then the gamma-Nd2S3 -> alpha-Nd2S3 transition occurs within 20 h. The microhardness of the phases is: alpha-Nd2S3 H = 451 +/- 4 HV; gamma-Nd2S3 H = 531 +/- 4 HV. It was found by the TG method that the Nd10S14O phase thermally dissociates at temperatures above 1400 degrees C. The mass loss is 0.5 mass % at 1580 degrees C and 1.0 mass % at 1620 degrees C, but the samples remain single-phase ones after cooling. However, two impurity phases gamma-Nd2S3-x and Nd2O2S appear in the Nd10S14O samples treated at temperatures above 1620 +/- 20 degrees C. For samples of the Nd10S14O phase annealed in an argon atmosphere at temperatures of 1050, 1400, 1580 degrees C, a regular decrease in the unit cell parameters and optical band gap was recorded: 1050 degrees C a = 15.06291(28), c = 19.97864(35), E-g = 2, 63 eV, 1400 degrees C a = 15.04779(36), c = 19.97160(44), E-g = 2.64 eV; 1580 degrees 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 degrees C to the melt was constructed. The Nd2O2S phase melts congruently at 2050 +/- 30 degrees C. Eutectics with coordinates 23 mol. % Nd2O3 (0.3484 Nd10S14O + 0.6516 Nd2O2S), t = 1553 +/- 1.8 degrees C; Delta H = 187 +/- 19 J/g; 82 mol. % Nd2O3; (0.54 Nd2O2S + 0.46 Nd2O3), t = 1970 +/- 30 degrees C 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 Delta H = 67 kJ/mol was calculated using the Schroeder equation.