Facile Oxide to Chalcogenide Conversion for Actinides Using the Boron–Chalcogen Mixture Method

Actinide chalcogenides
are of interest for fundamental studies
of the behavior of 5f electrons in actinides located in a soft ligand
coordination environment. As actinides exhibit an extremely high affinity
for oxygen, the synthesis of phase-pure actinide chalcogenide materials
free of oxide impurities is a great challenge and, moreover, requires
the availability and use of oxygen-free starting materials. Herein,
we report a new method, the boron–chalcogen mixture (BCM) method,
for the synthesis of phase-pure uranium chalcogenides based on the
use of a boron–chalcogen mixture, where boron functions as
an “oxygen sponge” to remove oxygen from an oxide precursor
and where the elemental chalcogen effects transformation of the oxide
precursor into an oxygen-free chalcogenide reagent. The boron oxide
can be separated from the reaction mixture that is left to react to
form the desired chalcogenide product. Several syntheses are presented
that demonstrate the broad functionality of the technique, and thermodynamic
calculations that show the underlying driving force are discussed.
Specifically, three classes of chalcogenides that include both new
(rare earth uranium sulfides and alkali–thorium thiophosphates)
and previously reported compounds were prepared to validate the approach:
binary uranium and thorium sulfides, oxide to sulfide transformation
in solid-state reactions, and in situ generation of actinide chalcogenides
in flux crystal growth reactions.