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

Complex perovskite materials have attracted extensive attention due to their fascinating properties and novel features attributed to the coexistence of ferro/ferri/antiferro-magnetic ground state and semiconductor band gap within the same material. Triple perovskite Sr3Co2WO9 has been successfully synthesized for the first time in the nanocrystalline form with an average crystallite size of 23 nm by means of a modified 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 formation of Sr3Co2WO9 was studied in situ by X-ray diffraction and analysed by the Rietveld analysis. The detected hysteresis loops with non-zero remanent magnetization and rather large coercive field reveal ferrimagnetic ordering with Curie temperature of 144 K. The measured effective magnetic moment of 3B is close to the expected value for rarely observed intermediate spin S = 1.It is found that the compound exhibits the semiconducting properties with the optical band gaps equal to 3.52 eV (indirect) and 3.76 eV (direct), respectively, further confirmed by AC conductivity measurements, which varies between 10−5 and 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 = W, Te) 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 if 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 of both compounds confirms the non-Debye type of relaxation behavior and the dominance of shorter-range movement of charge carriers. Magnetic studies for both compounds revealed antiferromagnetic behavior with TN being 57 K for SNWO and 35 K for SNTO.

perovskites ; nanocrystalline ; sol-gel ; magnetic ; dielectric

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