Thermodynamics of iron sulfides II. Heat capacity and thermodynamic properties of FeS and of Fe0.875S at temperatures from 298.15 K to 1000 K, of Fe0.98S from 298.15 K to 800 K, and of Fe0.89S from 298.15 K to about 650 K. Thermodynamics of formation

The heat capacities of FeS and Fe 7 S 8 have been measured by adiabatic-shield calorimetry at temperatures T from 298.15 K to 1000 K. In addition, heat capacities of Fe 0.98 S and Fe 0.89 S have been determined from T = 298.15 K to about 800 K and 650 K, respectively. All compounds crystallize in (NiAs or Cd(OH) 2 )-related types of structure, and they all exhibit transitions due to disappearance of the lower-temperature antiferromagnetic or ferrimagnetic phase. The Néel temperature is near 590 K for all four samples. FeS shows two additional transitions with heat-capacity maxima at T = 419.6 K and 440 K. The lower-temperature transition originates from structural changes, whereas the higher one is mainly of magnetic origin. For Fe 0.98 S only one additional transition takes place, with maximum heat capacity at T = 405 K. So also for Fe 0.89 S, which exhibits a transition 30 K below the Néel temperature. The maximum heat capacity at T = 560 K is due to a structural transition coupled to a magnetic-order-to-order transition. The structural-order-to-disorder transition and the magnetic one overlap in Fe 7 S 8 , giving rise to one heat-capacity peak only in the Néel-temperature region. In addition, a smaller effect, related to a phase reaction, is observed in the range T = 650 K to 760 K. The thermodynamic properties and the formation functions are evaluated and discussed.