Cerium Substitution in Yttrium Iron Garnet: Valence State, Structure, and Energetics

Abstract

The garnet structure is a promising nuclear waste form because it can accommodate various actinide elements. Yttrium iron garnet, Y 3 Fe 5 O 12 (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 3−x Ce x Fe 5 O 12 with 0 ≤ x ≤ 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 57 Fe− Mossbauer 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 Cesubstituted 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-U corrections, which reproduce the observed increase in the tendency for tetravalent Ce to be present with a higher loading of Ce. The DFT+U 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.

First principles calculation on the magnetic, optical properties and oxygen vacancy effect of CexY3−xFe5O12

“…(7) in Ref. 38). The higher Ce 3þ population in oxygen vacancy incorporated Ce 0.125 Y 2.875 Fe 5 O 12 system can lead to stronger Faraday rotation of this material as observed experimentally.…”

“…Recently, it was found that Ce 3þ would change to Ce 4þ at room temperature due to its larger ionic radius compared to Y 3þ . 38 This process seemed to be counteracted by annealing Ce:YIG thin films or single crystals in a low oxygen partial pressure ambient, which lead to higher ε ε near infrared Faraday rotation of Ce:YIG. 17,18 To understand the experimental observations, we calculated the formation enthalpy difference (DH) between Ce 3þ -Fe 3þ and Ce 4þ -Fe 2þ valence states using the strategy described in Ref.…”

“…17,18 To understand the experimental observations, we calculated the formation enthalpy difference (DH) between Ce 3þ -Fe 3þ and Ce 4þ -Fe 2þ valence states using the strategy described in Ref. 38. The one oxygen vacancy was placed at different distances from the Ce ion (d Ce-VO ) in a Ce 0.125 Y 2.875 Fe 5 O 12 unit cell.…”

Photoactuation Healing of α‐FeOOH@g‐C3N4 Catalyst for Efficient and Stable Activation of Persulfate

“…XANES spectroscopy was at the meantime employed to determine the change in the valence state of Fe in 5%‐α‐FeOOH@carbon‐ring‐C 3 N 4 after light excitation. The spectrum of the Fe(II) reference, shown in Figure c exhibited the contribution of a characteristic XANES peak for divalent Fe at ≈7120 eV, due to the 1s→4p electron transition . In contrast, the trivalent iron peak was shifted to higher energy at 7127 eV.…”

Study on Thermodynamic Properties of Fe3+-Substituted Yttrium Iron Garnets

“…One can locate only a limited number of articles bearing on studies of the thermodynamic properties of pure and substituted Y 3 Fe 5 O 12 garnet ferrites [6][7][8]. Recently, the thermodynamic stability of Th 4+ -doped yttrium iron garnet system, Y 3−x Th x Fe 5 O 12 (x = 0.04−0.07), as a possible actinide bearing material has been investigated using calorimetric measurements and first principles electronstructure calculations [9].…”