Preparation and study of elemental californium-249

Bulk samples of 249Cf metal were prepared on the 10 μg scale via the Li metal vapor reduction of 249CfF3. Above about 7250C elemental Cf exhibits a face-centered cubic (fcc) structure with an average, room-temperature lattice parameter of 5.75(1)A. Between about 6000C and 7250C, the stable form of Cf metal is another fcc structure with an average, room-temperature lattice parameter of 4.94(1)A. Below 6000C metallic Cf exhibits a double hexagonal closest packed (dhcp) structure with average, room temperature lattice parameters of a0 = 3.39(1)A and c0 = 11.01(5)A. By comparison of the metallic radii calculated for these three forms with those of the receding transuranium elements, it is suggested that the two, lower temperature modifications represent Cf with a metallic valence of three, while the highest temperature form represents a metallic valence of two. Although the data reported here are from the most complete study to date of elemental Cf, the limitations accompanying such microscale research are duly noted. (U.S.)