Electronic Structure and Properties of Berkelium Iodates

The reaction of ²⁴⁹Bk(OH)₄ with iodate under hydrothermal conditions results in the formation of Bk(IO₃)₃ as the major product with trace amounts of Bk(IO₃)₄ also crystallizing from the reaction mixture. The structure of Bk(IO₃)₃ consists of nine-coordinate Bkᴵᴵᴵ cations that are bridged by iodate anions to yield layers that are isomorphous with those found for Amᴵᴵᴵ, Cfᴵᴵᴵ, and with lanthanides that possess similar ionic radii. Bk(IO₃)₄ was expected to adopt the same structure as M(IO₃)₄ (M = Ce, Np, Pu), but instead parallels the structural chemistry of the smaller Zrᴵⱽ cation. Bkᴵᴵᴵ–O and Bkᴵⱽ–O bond lengths are shorter than anticipated and provide further support for a postcurium break in the actinide series. Photoluminescence and absorption spectra collected from single crystals of Bk(IO₃)₄ show evidence for doping with Bkᴵᴵᴵ in these crystals. In addition to luminescence from Bkᴵᴵᴵ in the Bk(IO₃)₄ crystals, a broad-band absorption feature is initially present that is similar to features observed in systems with intervalence charge transfer. However, the high-specific activity of ²⁴⁹Bk (t₁/₂ = 320 d) causes oxidation of Bkᴵᴵᴵ and only Bkᴵⱽ is present after a few days with concomitant loss of both the Bkᴵᴵᴵ luminescence and the broadband feature. The electronic structure of Bk(IO₃)₃ and Bk(IO₃)₄ were examined using a range of computational methods that include density functional theory both on clusters and on periodic structures, relativistic ab initio wave function calculations that incorporate spin–orbit coupling (CASSCF), and by a full-model Hamiltonian with spin–orbit coupling and Slater–Condon parameters (CONDON). Some of these methods provide evidence for an asymmetric ground state present in Bkᴵⱽ that does not strictly adhere to Russel–Saunders coupling and Hund’s Rule even though it possesses a half-filled 5f ⁷ shell. Multiple factors contribute to the asymmetry that include 5f electrons being present in microstates that are not solely spin up, spin–orbit coupling induced mixing of low-lying excited states with the ground state, and covalency in the Bkᴵⱽ–O bonds that distributes the 5f electrons onto the ligands. These factors are absent or diminished in other f⁷ ions such as Gdᴵᴵᴵ or Cmᴵᴵᴵ.