Synthesis, X-ray Rietveld analysis, infrared and Mossbauer spectroscopy of R2FeSbO7 (R3+ = Y, Dy, Gd, Bi) pyrochlore solid solution

A few iron-antimonate pyrochlores R2FeSbO7 (R3+ = Y, Dy, Gd, Bi) were synthesized using the solid state reaction technique and characterized systematically through powder X-ray diffraction and its Rietveld analysis, SEM-EDAX, Fourier-transform infrared and Fe-57 Mossbauer spectroscopy. Scherrer's analysis of the XRD pattern and SEM-EDAX analysis estimate particle size of the polycrystalline powder to be in the range of 30-40 nm. Using the Rietveld fitting of the XRD profile, a model for unit Bravais cell is constructed suitably choosing 4 R ions, 4 Fe/Sb ions, 12 O ions and 2 O' ions. A very strong and broad band at high wavenumber end at 642.0 cm(-1) followed by some low intensity bands at 490, 465, 447 and 432 cm(-1) are observed in the mid-infrared region of 400-1000 cm(-1) in FT-IR spectra of Y2FeSbO7. The bands at 642 and 490 cm(-1) can be ascribed to the stretching vibrations of the smallest inter-atomic bond M(Fe/Sb)-O in MO6 octahedron and Y-O' bond, respectively. Mossbauer spectra exhibit paramagnetic doublet and sextet magnetic splitting at 300 K and 9 K, respectively. Quadrupole splitting measured at 300 K decreases linearly with the decrease of the x-coordinate of the 48f oxygens, which measures the distortion of the FeO6 octahedron. The quadrupole splitting arises due to the electric field gradient at Fe3+ nucleus site, which is nearly one-half of the value found for high-spin Fe2+ nucleus. Quadrupole splitting in the magnetic ordered spectra taken at 9 K is reduced from its 300 K value due to random distribution of the hyperfine fields at Fe site with regard to the electric field gradient axis. The larger effective magnetic moment of cations causes higher hyperfine field at Fe nucleus and vice-versa in these compounds. (C) 2015 Elsevier B.V. All rights reserved.