2 research outputs found
Electronic Structure and Properties of Berkelium Iodates
The
reaction of <sup>249</sup>BkÂ(OH)<sub>4</sub> with iodate under
hydrothermal conditions results in the formation of BkÂ(IO<sub>3</sub>)<sub>3</sub> as the major product with trace amounts of BkÂ(IO<sub>3</sub>)<sub>4</sub> also crystallizing from the reaction mixture.
The structure of BkÂ(IO<sub>3</sub>)<sub>3</sub> consists of nine-coordinate
Bk<sup>III</sup> cations that are bridged by iodate anions to yield
layers that are isomorphous with those found for Am<sup>III</sup>,
Cf<sup>III</sup>, and with lanthanides that possess similar ionic
radii. BkÂ(IO<sub>3</sub>)<sub>4</sub> was expected to adopt the same
structure as MÂ(IO<sub>3</sub>)<sub>4</sub> (M = Ce, Np, Pu), but instead
parallels the structural chemistry of the smaller Zr<sup>IV</sup> cation.
Bk<sup>III</sup>–O and Bk<sup>IV</sup>–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 BkÂ(IO<sub>3</sub>)<sub>4</sub> show
evidence for doping with Bk<sup>III</sup> in these crystals. In addition
to luminescence from Bk<sup>III</sup> in the BkÂ(IO<sub>3</sub>)<sub>4</sub> 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 <sup>249</sup>Bk (<i>t</i><sub>1/2</sub> = 320 d) causes oxidation of
Bk<sup>III</sup> and only Bk<sup>IV</sup> is present after a few days
with concomitant loss of both the Bk<sup>III</sup> luminescence and
the broadband feature. The electronic structure of BkÂ(IO<sub>3</sub>)<sub>3</sub> and BkÂ(IO<sub>3</sub>)<sub>4</sub> were examined using
a range of computational methods that include density functional theory
both on clusters and on periodic structures, relativistic <i>ab initio</i> 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<sup>IV</sup> that does not strictly adhere to Russel–Saunders
coupling and Hund’s Rule even though it possesses a half-filled
5<i>f</i> <sup>7</sup> shell. Multiple factors contribute
to the asymmetry that include 5<i>f</i> 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<sup>IV</sup>–O bonds that distributes
the 5<i>f</i> electrons onto the ligands. These factors
are absent or diminished in other <i>f</i><sup>7</sup> ions
such as Gd<sup>III</sup> or Cm<sup>III</sup>