Mössbauer spectral evidence for rhombohedral symmetry in R₃Fe₅O₁₂ garnets with R = Y, Eu and Dy

The iron-57 Mössbauer spectra of R₃Fe₅O₁₂, where R is Y, Eu and Dy, have been measured between 4.2 and 550 K. The substantial quadrupole splittings observed in the paramagnetic spectra confirm that the local symmetry at both the tetrahedral and octahedral iron(III) sites is not cubic. The low temperature Mössbauer spectra of Dy₃Fe₅O₁₂ clearly confirm the spin reorientation between 10 and 15 K and the 4.2 and 10 K spectra are consistent with the known orientation of the magnetization at 14 K in the cubic Iad unit cell. The Mössbauer spectra of R₃Fe₅O₁₂, where R is Y, Eu and Dy, obtained between 45 and 295 K, reveal four different tetrahedral iron(III) Mössbauer spectral components, four components which are inconsistent with a magnetization oriented along the [111] axis of a cubic Iad unit cell. In contrast, these four components are consistent with a crystal symmetry which is reduced from cubic to rhombohedral R. The temperature dependence of the hyperfine fields in Dy₃Fe₅O₁₂ indicates a small biquadratic exchange contribution to the magnetic exchange. The temperature dependence of the isomer shifts in Dy₃Fe₅O₁₂ gives Mössbauer lattice temperatures of 405 and 505 K for the 16a and 24d sites, respectively, values which are in excellent agreement with the Debye temperature measured for Y₃Fe₅O₁₂.