Local coordination environment in coupled Mn2+/Si4+ substituted Gd3Fe5O12 garnet: Application as a waste form for intermediate-level nuclear waste

Mn and Si have been successfully incorporated into the garnet structure of Gd 3 Fe 5 O 12 via co-substitution at the Fe sublattice in the structure. A series of samples were prepared with the general formula Gd 3 Mn x Si x Fe 5-2x O 12 (x = 0.01, 0.02, 0.04, 0.08, 0.10, and 0.20). Single-phase garnet samples were prepared by heating stoichiometric amounts of the oxides of precursors at 1400 0 C for 72 hours. X-ray diffraction confirmed the formation of predominantly single-phase garnet across all compositions, with no detectable secondary phases. With increasing concentration of Mn 2+ /Si 4+ , the lattice parameter (a) and the unit cell volume decreased from 1939.1 Å 3 (a = 12.47 Å) for the prototype Gd 3 Fe 5 O 12 to 1933.8 Å 3 (a = 12.45 Å) for Gd 3 Mn 0.2 Si 0.2 Fe 4.6 O 12 . A decrease in the crystallite size from 59.0 ± 4.0 nm for Gd 3 Fe 5 O 12 to 48.6 ± 2.0 nm for Gd 3 Mn 0.2 Si 0.2 Fe 4.6 O 12 was also observed. X-ray absorption near edge structure (XANES) spectroscopy was used to probe the oxidation states and local coordination environment of the Fe and Mn sites. The oxidation state of Fe in Gd 3 Fe 5 O 12 is 3+; therefore, during co-substitution, two Fe 3+ ions are replaced by one Mn 2+ ion and one Si 4+ ion. For Extended X-ray Absorption Fine Structure (EXAFS) analysis, the structural model for Fe was fit from 0.3 Å – 3.5 Å in R-space. EXAFS analysis indicated that Fe atoms occupy both corner-sharing octahedra and tetrahedra sites in Gd 3 Mn x Si x Fe 5-2x O 12 . To account for the Mn scattering paths, the structural model was fit up to 3.5 Å in R-space. The local environment of the Mn atoms suggests the presence of a positively charged defect in proximity to negatively charged defects. Further, Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was employed to confirm successful and homogenous incorporation of Mn and Si into the garnet lattice. The measured concentrations of Mn and Si increase progressively with increasing x, ranging for Mn as 2542.6 ppm for x=0.01 to 16677.2 ppm for x=0.2 and for Si as 4360.7 ppm for x=0.01 to 10612.1 ppm for x=0.2. Overall, the combined characterization techniques such as XRD, XANES, EXAFS, and LA-ICP-MS confirmed successful coupled Mn and Si substitution in Gd 3 Fe 5 O 12 with preserved garnet structure and a charge-balanced defect mechanism governing local atomic arrangement.