Vapor Pressure and Crystal Structure of Curium Metal

The vapor pressure of liquid and solid curium metal between 1179° and 2068°K and between 10−10 atm and 0.1 mm was measured by the Knudsen effusion method to be log10P(atm) = (4.74 ± 0.37)–(18 060 ± 590) / T(°K); ΔH°1600°K and ΔS°1600°K for vaporization are 82.6 ± 2.7 kcal/g·atm and 21.7 ± 1.7 eu, respectively. A previously unreported fcc close-packed structure for curium metal was identified with a = 4.382 ± 0.004 Å, and a density of 19.26 g/cm3, corresponding to a +4 metallic valence. The entropy of condensed curium is indicated to be relatively high by the low heat of fusion and by the ΔH°298°K for vaporization calculated by second- and third-law methods. The heat of vaporization of curium may be explained from the opposing effects of a quadrivalent metal and a 5f66d27s2 to 5f76d7s2 transition on vaporization.