The vapor pressure of high-purity solid and liquid curium

The vapor pressure of triply−distilled curium−244 metal has been measured in the temperature range 1300−2000 K, using Knudsen effusion target−collection techniques. Precise geometrical conditions were chosen to insure that the intercepted molecular beam came entirely from the curium sample surface within the cell. Second−law equations are Log10P (atm) = (6.082±0.129) − (19618±193)/T (K) (solid, 1327−1639 (K), Log10P (atm) = (5.586±0.157) − (18894±275)/T (K) (liquid, 1640−1972 (K). From these data, the heat and entropy of fusion/transition are calculated to be 3.31 kcal and 2.19 cal/deg, respectively. The average second−law entropy is 25.56±0.72 cal/deg. Similarities noted in other work between the chemical and physical properties of curium and gadolinium were clearly evident. Free−energy functions for gadolinium were used directly to give ΔH298K (3rd−law) = 92128±76 cal for the heat of sublimation from the solid phase, and 92482±112 cal for the liquid. Difficulties encountered in high−temperature effusion studies with reactive materials are discussed in the light of these and earlier results.