Electronic Structure and Properties of Berkelium Iodates

Abstract

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 Bk^III cations that are bridged by iodate anions to yield layers that are isomorphous with those found for Am^III, Cf^III, 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 Zr^IV cation. Bk^III-O and Bk^IV-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 Bk^III in these crystals. In addition to luminescence from Bk^III 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 (t_1/2 = 320 d) causes oxidation of Bk^III and only Bk^IV is present after a few days with concomitant loss of both the Bk^III 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 Bk^IV 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 Bk^IV-O bonds that distributes the 5f electrons onto the ligands. These factors are absent or diminished in other f⁷ ions such as Gd^III or Cm^III.