Enthalpy of Formation of a Solid Soluion in the Cr-S System

Abstract

Enthalpy changes of combustion (ΔH_c) of nonstoichiometric chromium sulfides; trigonal-Cr₂S₃ (t-Cr₂S₃) and rhombohedral-Cr₂S₃ (r-Cr₂S₃), were measured by using a Tian-Calvet type twin-calorimeter. The enthalpy change of phase transformation from t-Cr₂S₃ to r-Cr₂S₃ (CrS_1.471 to CrS_1.478) was found to be; ΔH_T/kJ·mol^-1=-83.16. In the nonstoichiometric t-Cr₂S₃ phase (CrS_1.402-CrS_1.471), the ΔH_c value stayed unchanged at about-840kJ·mol^-1 in the compositional range of CrS_1.450-CrS_1.471, while it increased with decreasing sulfur composition (x in CrS_x) up to c. a. -420kJ·mol^-1 at CrS_1.402, suggesting that the crystal lattice of CrS_x becomes more stable at lower sulfur compositions in the t-Cr₂S₃ phase. The enthalpies of the foration of solid slution (ΔH^ss)in the t-Cr₂S₃ phase were also estimated by assuming CrS_1.478 (sulfurrich end of the t-Cr₂S₃ phase) as the standard state and that the more sulfur deficient compositions than CrS_1.478 were considered to be formed by the dissolution of Cr atoms into CrS_1.478. ΔH^ss was almost zero in the compositional range of CrS_1.450-CrS_1.471, while it increased with decreasing sulfur composition x in x < 1.450. It went through a maximum of c. a. 370kJ·mol^-1 at CrS_1.440 and then decreased.