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Sol-Gel-Based Synthesis Design and Magnetic and Dielectric Properties Study of Selected Nanocrystalline Double and Triple Perovskites

Complex perovskites have attracted extensive attention due to their fascinating physical properties and novel features owing to the coexistence of the ferro/ferri-magnetic ground state and semiconducting behavior in the single material. The triple perovskite Sr3Co2WO9(SCWO) has been successfully synthesized for the first time in the nanocrystalline form with an average crystallite size of 23 nm using a high yield (81 %) aqueous citrate sol-gel method. At room temperature, the crystal structure of Sr3Co2WO9 is cubic, space group Fm-3m, with lattice parameter a= 7.9073(6) Å. The detected hysteresis loops with non-zero remanent magnetization and rather large coercive field reveal ferrimagnetic ordering with a Curie temperature of 144 K. The measured effective magnetic moment of 3 µBis close to the expected value for rarely observed intermediate spin S = 1. It is found that the compound exhibits semiconducting properties with the optical band gaps equal to 3.52 eV (indirect) and 3.76 eV (direct), respectively, further confirmed by the determination of the AC conductivity, which in the measured temperature range (25 -500 °C at 1 kHz) lies within the interval from 10−5-10−4 Ω−1 cm−1. The Maxwell -Wagner model is employed to describe the frequency dependent dielectric constant. The frequency-dependent AC conductivity follows the universal Jonscher’s power law. Double perovskites with Sr2NiMO6 (M = Te, W) structure type have been similarly synthesized. The reaction yielded phase pure nanocrystalline powders of two compounds Sr2NiWO6 (SNWO) and Sr2NiTeO6 (SNTO). According to the Rietveld refinement of powder X-ray diffraction data at room temperature Sr2NiWO6 is tetragonal (I4/m) and Sr2NiTeO6 is monoclinic (C12/m1) with average crystallite sizes of 49 and 77 nm, respectively. Both SNTO and SNWO possess high values of dielectric constants (341 and 308, respectively) with low dielectric loss (0.06 for SNWO) at frequency of 1 kHz. The Nyquist plot for both samples confirms the non-Debye type of relaxation behavior and the dominance of shorter-range movement of charge carriers. Magnetic studies of both compounds revealed antiferromagnetic behavior with TN being 57 K for SNWO and 35 K for SNTO.

perovskites ; sol-gel ; magnetism ; dielectrics

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