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.