Knudsen Cell Studies of the Vaporization of Samarium Dicarbide

The vapor pressure of SmC₂ in equilibrium with graphite was measured by the Mnudsen effusion technique. Rates of weight loss from the cells were measured with an automatic recording balance. The apparent pressures varied with orifice size, and equilibrium pressures were calculated by extrapolation to zero orifice area. This work was combined with other studies to obtain log₁₀ P(atm) = - 13.869 × 10³/T + 3.752 (1300°-2050°K) for the Sm vapor pressure above SmC₂-C. Estimates of S°₂₉₈ and c_p were made for SmC₂, and δH°₂₉₈ was calculated to be 72.0 ± 2 kcal/mol for the reaction SmC₂(s) = Sm(g) + 2C(s). This value combined with δH°_{v, 298}= 48.6 kcal/mol for Sm gives a δ°_f298 for SmC₂ of 23.4 ± 2 kcal/mol.