Synthesis, structure, UV-Vis-NIR, infrared and Raman spectroscopy, and force-field investigation for A2GaSbO7 (A3+Â = Y, Dy, Gd) pyrochlores

Three mixed pyrochlores, A2GaSbO7 (A3+ = Y, Dy, Gd) were synthesized using the solid-state reaction technique and characterized systematically through powder X-ray diffraction and its Rietveld analysis, SEM-EDAX, UV-Visible absorption spectra, Fourier-transform infrared and Raman spectroscopy. Scherrer analysis of the XRD pattern and SEM-image were used to estimate size of the nano-structured particles of the polycrystalline samples to be ∼40-50 nm. From the UV-Vis absorbance data, absorption band edge and optical band gap were determined, and found to be ∼350 ± 8 nm and 2-2.5 eV, respectively, for these compounds. Four F1u bands were only observed in the infrared spectra within the phonon wave numbers range of 400-800 cm−1 for these samples. The highest wave number infrared mode arises due to the interaction between bending and stretching of A−O and B−O bonds. A mode-splitting caused by the mass-difference of B-site ions was found in the other three infrared modes. The Raman spectra of these compounds exhibit six Raman modes, e.g., A1g, Eg, and 4F2g, within the experimental range of 50-1000 cm−1, with few extra bands in the higher (>700 cm−1) and lower (<200 cm−1) wave numbers range. A normal coordinate analysis using a generalized valence force-field model has been performed to assign and evaluate the vibration modes in these compounds. Five stretching and four bending force constants were considered to construct the force-field matrix. Comparison of the calculated phonon wave numbers with experimental values obtained in infrared and Raman excitation zones describes excellent agreement for all these isomorphous compounds.