Mössbauer Spectral Properties of Yttrium Iron Garnet, Y₃Fe₅O₁₂, and Its Isovalent and Nonisovalent Yttrium-Substituted Solid Solutions

Several high-resolution Mössbauer spectra of yttrium iron garnet, Y₃Fe₅O₁₂, have been fit as a function of temperature with a new model based on a detailed analysis of the spectral changes that result from a reduction from the cubic Ia3d space group to the trigonal R3 space group. These spectral fits indicate that the magnetic sextet arising from the 16a site in cubic symmetry is subdivided into three sextets arising from the 6f, the 3d, 3d, and the 1a, 1b, 2c sites in rhombohedral-axis trigonal symmetry. The 24d site in cubic Ia3d symmetry is subdivided into four sextets arising from four different 6f sites in R3 rhombohedral-axis trigonal symmetry, sites that differ only by the angles between the principal axis of the electric field gradient tensor and the magnetic hyperfine field assumed to be parallel with the magnetic easy axis. This analysis, when applied to the potential nuclear waste storage compounds Y_3-xCa_0.5xTh_0.5xFe₅O₁₂ and Y_3-xCa_0.5xCe_0.5xFe₅O₁₂, indicates virtually no perturbation of the structural, electronic, and magnetic properties upon substitution of small amounts of calcium(II) and thorium(IV) or cerium(IV) onto the yttrium(III) 24c site as compared with Y₃Fe₅O₁₂. The observed broadening of the four different 6f sites derived from the 24d site results from the substitution of yttrium(III) with calcium(II) and thorium(IV) or cerium(IV) cations on the next-nearest neighbor 24c site. In contrast, the same analysis applied to Y_2.8Ce_0.2Fe₅O₁₂ indicates a local perturbation of the magnetic exchange pathways as a result of the presence of cerium(IV) in the 24c next-nearest neighbor site of the iron(III) 24d site.