Characterization of nanocrystalline triple perovskites synthesized by a novel modified sol–gel route instead of bulk materials synthesized by a solid-state route.
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. Herein, the triple perovskite Sr 3 Co 2 WO 9 (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 Sr 3 Co 2 WO 9 is cubic, space group Fm3̅ m, with lattice parameter a = 7.9073(6) Å. The formation of SCWO triple perovskite was studied in situ by X-ray diffraction and subsequently analyzed by the Rietveld analysis. The detected hysteresis loops with nonzero remanent magnetization and rather large coercive field reveal ferrimagnetic ordering with a Curie temperature of 144 K. The measured effective magnetic moment of 3 μ B is close to the expected value for the 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 power law. Since it possesses both magnetic and semiconductor properties, this material could be a promising candidate to use in devices where its semiconducting properties would be spin-controlled.
Double perovskites have been extensively studied in materials chemistry due to their excellent properties and novel features attributed to the coexistence of ferro/ferri/antiferro-magnetic ground state and semiconductor band gap within the same material. Double perovskites with Sr2NiMO6 (M = Te, W) structure type have been synthesized using simple, non-toxic and costless aqueous citrate sol-gel route. 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. Structural studies have been additionally performed by Raman spectroscopy revealing optical phonons typical for vibrations of Te6+/W6+O6 octahedra. Both SNTO and SNWO possess high values of dielectric constants (341 and 308, respectively) with low dielectric loss (0.06 for SNWO) at a frequency of 1 kHz. These values decrease exponentially with the increase of frequency to 1000 kHz, with the dielectric constant being around 260 for both compounds and dielectric loss being 0.01 for SNWO and 0.04 for SNTO. 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 Néel temperature (TN) being 57 K for SNWO and 35 K for SNTO.
The main objective of this work was to synthesize phase pure double perovskites Ba2NiTeO6 (BNTO) and Ba2NiWO6 (BNWO) in nanocrystalline form and to reveal the impact of nanocrystallinity onto their magnetic and dielectric properties.
The studied double perovskites were synthesized in nanocrystalline form by employing citrate sol-gel route. A detailed investigation of their structure and properties using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy technique, energy-dispersive X-ray spectroscopy (EDS), SQUID magnetometry and electrical measurements is carefully described.
Rietveld refinement of XRD patterns revealed phase purity of both compounds, BNTO is trigonal (R-3m) while BNWO is cubic (Fm-3m). Raman spectroscopy studies reveal optical phonons that correspond to vibrations of Te6+/W6+O6 octahedra, while SEM images show irregular plate-like nanocrystals. Magnetic property measurements speak in favor of antiferromagnetic order but in both compounds size reduction affected their properties. BNTO has Néel temperature (TN) of 10.3 K which is higher than previously reported for its bulk form. Magnetic ground state of BNWO can be explained as canted antiferromagnetism with TN = 48.2 K. Room temperature measurements of dielectric constants at various frequencies suggest that these materials are high-κ dielectrics with low dielectric loss. The Nyquist plot reveals depressed semicircle arc typical for non-Debye type of relaxation phenomena for BNWO ceramic, whereas for BNTO ceramic almost straight line of Zʹʹ versus Z’ has been observed indicating its high insulating behavior.
To conclude, size-dependent properties of studied double perovskites are discussed introducing a possibility for implementation in electronic devices.
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