1 research outputs found
Cerium Substitution in Yttrium Iron Garnet: Valence State, Structure, and Energetics
The garnet structure is a promising
nuclear waste form because
it can accommodate various actinide elements. Yttrium iron garnet,
Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub> (YIG), is a model composition
for such substitutions. Since cerium (Ce) can be considered an analogue
of actinide elements such as thorium (Th), plutonium (Pu), and uranium
(U), studying the local structure and thermodynamic stability of Ce-substituted
YIG (Ce:YIG) can provide insights into the structural and energetic
aspects of large ion substitution in garnets. Single phases of YIG
with Ce substitution up to 20 mol % (Y<sub>3–<i>x</i></sub>Ce<sub><i>x</i></sub>Fe<sub>5</sub>O<sub>12</sub> with 0 ≤ <i>x</i> ≤ 0.2) were synthesized
through a citrate–nitrate combustion method. The oxidation
state of Ce was examined by X-ray absorption near edge structure spectroscopy
(XANES); the oxidation state and site occupancy of iron (Fe) as a
function of Ce loading also was monitored by <sup>57</sup>Fe–Mössbauer
spectroscopy. These measurements establish that Ce is predominantly
in the trivalent state at low substitution levels, while a mixture
of trivalent and tetravalent states is observed at higher concentrations.
Fe was predominately trivalent and exists in multiple environments.
High temperature oxide melt solution calorimetry was used to determine
the enthalpy of formation of these Ce-substituted YIGs. The thermodynamic
analysis demonstrated that, although there is an entropic driving
force for the substitution of Ce for Y, the substitution reaction
is enthalpically unfavorable. The experimental results are complemented
by electronic structure calculations performed within the framework
of density functional theory (DFT) with Hubbard-<i>U</i> corrections, which reproduce the observed increase in the tendency
for tetravalent Ce to be present with a higher loading of Ce. The
DFT+<i>U</i> results suggest that the energetics underlying
the formation of tetravalent Ce involve a competition between an unfavorable
energy to oxidize Ce and reduce Fe and a favorable contribution due
to strain-energy reduction. The structural and thermodynamic findings
suggest a strategy to design thermodynamically favorable substitutions
of actinides in the garnet system