Enthalpy and Gibbs energy of formation of samarium dicarbide

Enthalpy and Gibbs energy of formation of SmC2 have been determined between 1420 and 1650 K by measuring the CO(g) partial pressure over the invariant SmC2(s)–SmO1.5(s)–C(s) three-phase field. The equilibrium CO pressure over the three-phase field was deduced from the effusion pressure by the dynamic effusion MS method. The Gibbs energy of formation of SmC2(s) in the entire temperature range was deduced from the Gibbs energy of the reaction, SmO1.5(s) + 3.5 C(s)=SmC2(s) + 1.5 CO(g), and by taking the Gibbs energy functions of the other constituents from literature. The recommended Gibbs energy of formation of SmC2 at 298 K is −(98 ± 7) kJ mol−1. The enthalpy of formation of SmC2(s) was derived from the enthalpy of reaction and the enthalpies of formation of SmO1.5(s) and CO(g) taken from literature. The third-law enthalpy of formation of SmC2(s) at 298 K is −(85 ± 8) kJ mol−1.