Optical and Magnetic Properties of Cubic Double Perovskites Ba2RSbO6 (R= Dy, Gd) Coordinated to Lattice Dynamical and Crystal-Field Computations

Herein, optical (e.g., UV-Vis reflectance spectra, infrared and Raman spectroscopy) and magnetization properties of the double-perovskite compounds Ba2DySbO6 (BDS) and Ba2GdSbO6 (BGS) are reported. Optical bandgap is determined to be 4.524 eV for BDS and 4.465 eV for BGS using Kubelka-Munk function fit to the UV-Vis spectra. The lattice dynamical calculations on the force-field model are undertaken for assigning the observed infrared and Raman modes. The dc susceptibility does not exhibit magnetic ordering down to 2 K for the two compounds. The dc magnetization at T = 2 K does not saturate for BDS even at a magnetic field of 11 T, but saturates at 5 T to a value of 7 mu(B)/Gd3+ for BGS. A frequency-dependent cusp in the ac susceptibility appears at 3.3 K, that is unlikely generic spin-glass-like freezing, and may be attributed to the short-range correlations among Dy3+-moments in BDS. Crystal-field (CF) computations are performed to analyze the magnetic data for BDS. The obtained CF level scheme of Dy3+ ions demonstrates the observed susceptibility and isothermal magnetization curves very well. The field-dependence of the two lowest CF levels along different principal axes does not exhibit any level crossing, consistent with the lack of ordering in Ba2DySbO6.