Vapor Pressure and Crystal Structure of Curium Metal

Abstract

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 $${\log }_{10}P\mathrm{(atm)}\text{\hspace{0.17em}=\hspace{0.17em}(4.74\hspace{0.17em}±\hspace{0.17em}0.37)–(18 060\hspace{0.17em}±\hspace{0.17em}590)\hspace{0.17em}/\hspace{0.17em}T(°K);}$$ ${\mathrm{\Delta H°}}_{\text{1600°K}}$ and ${\mathrm{\Delta S°}}_{\text{1600°K}}$ 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 $\text{a\hspace{0.17em}=\hspace{0.17em}4.382\hspace{0.17em}±\hspace{0.17em}0.004 Å}$ , 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 ${\mathrm{\Delta H°}}_{\text{298°K}}$ 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 ${\text{5f}}^6{\text{6d}}^2{\text{7s}}^2$ to ${\text{5f}}^7{\text{6d7s}}^2$ transition on vaporization.