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Magnetic, optical, dielectric, and sintering properties of nano-crystalline BaFe0.5Nb0.5O3 synthesized by a polymerization method

Köferstein, Roberto, Oehler, Florian, Ebbinghaus, Stefan G.
Journal of materials science 2018 v.53 no.2 pp. 1024-1034
temperature, X-ray diffraction, magnetism, glucose, polymerization, gels, shrinkage, crystallites, citric acid, powders, ceramics
A one-pot polymerization method using citric acid and glucose for the synthesis of nano-crystalline BaFe₀.₅Nb₀.₅O₃ is described. Phase evolution and the development of the crystallite size during decomposition of the (Ba,Fe,Nb)-gel were examined up to 1100 °C. Calcination at 850 °C of the gel leads to a phase-pure nano-crystalline BaFe₀.₅Nb₀.₅O₃ powder with a crystallite size of 28 nm. The shrinkage of compacted powders starts at 900 °C. Dense ceramic bodies (relative density ≥ 90%) can be obtained either after conventional sintering above 1250 °C for 1 h or after two-step sintering at 1200 °C. Depending on the sintering regime, the ceramics have average grain sizes between 0.3 and 52 µm. The optical band gap of the nano-sized powder is 2.75(4) eV and decreases to 2.59(2) eV after sintering. Magnetic measurements of ceramics reveal a Néel temperature of about 23 K. A weak spontaneous magnetization might be due to the presence of a secondary phase not detectable by XRD. Dielectric measurements show that the permittivity values increase with decreasing frequency and rising temperature. The highest permittivity values of 10.6 × 10⁴ (RT, 1 kHz) were reached after sintering at 1350 °C for 1 h. Tan δ values of all samples show a maximum at 1–2 MHz at RT. The frequency dependence of the impedance can be well described using a single RC-circuit.