Synthesis and characterization of BaSn(OH) 6 and BaSnO 3 acicular particles
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A. Martinelli Department of Chemistry and Industrial Chemistry, via Dodecaneso 31, I-16146 Genoa, Italy
A. Testino and P. Nanni Department of Process and Chemical Engineering, University of Genoa, Fiera del Mare, Pad. D., I-16146 Genoa, Italy (Received 3 May 2002; accepted 15 November 2002)
The synthesis of BaSn(OH)6 acicular crystals by precipitation at 100 °C from aqueous solutions and their transformation in the perovskitelike compound BaSnO3 was investigated. Single acicular crystals 100–200 m in length were obtained from a 0.05 M solution, whereas bundlelike aggregates of 20–40 m were precipitated from 0.2–0.6 M solutions. The x-ray diffraction pattern of barium hexahydroxostannate was indexed according to monoclinic symmetry with cell parameters a ⳱ 11.029 ± 0.002 Å, b ⳱ 6.340 ± 0.001 Å, c ⳱ 10.563 ± 0.001 Å,  ⳱ 128.51 ± 0.01°, ␣ ⳱ ␥ ⳱ 90°. The BaSn(OH)6 particles decomposed to BaSnO3 and water at approximately 270 °C and the original morphology was retained. The resulting product had specific surface area up to 30–40 m2/g and consisted of 10–20 nm crystallites. The larger unit cell edge in comparison to the reference value and the continuous weight loss up to 1200 °C indicate that water is not completely released during decomposition and a substantial amount of proton defects (up to 0.4 mol per mole of BaSnO3) is incorporated in the perovskite lattice as OH− groups. Normal crystallographic properties of BaSnO3 are restored only after calcination at 1300 °C. I. INTRODUCTION
In the last decade increasing attention has been paid to barium metastannate, BaSnO3, mainly for its use as gas, vapor, and humidity sensor.1–4 The crystal structure is that of a cubic perovskite with unit cell edge of 4.116 Å, and no phase transformations are reported up to the melting point, 2060 °C.5 Because of its high chemical and thermal stability, BaSnO3 can be potentially used at high temperature as a protective coating or catalyst support. The pure compound is an insulator at room temperature1,6 and becomes semiconducting when doped with donor impurities such as Sb5+ and La3+.7 When Sn4+ is heavily substituted with Y3+, the ceramic exhibits a high proton conductivity.8,9 Usually, ceramic powders of BaSnO3 are prepared by solid-state reaction between BaCO3 and SnO2 at 1000–1200 °C.7,10–12 Polycrystalline materials can be obtained by sintering at 1400–1600 °C, but good densification is difficult to achieve. Different
alternative routes for powder preparation have been explored: hydrothermal synthesis,13–15 precipitation from aqueous solutions,3,16,17 sol-gel techniques,18,19 the modified Pechini method,20 and the wet peroxide process.21 Among these techniques, precipitation from aqueous solutions seems particularly attractive because it uses rather inexpensive precursors such as SnCl4 and BaCl2. The precipitation occurs at high pH in the presence of NaOH and leads to the formation of the hexahydroxostannate compound BaSn(OH)6. The perovskite phase can be obtained by thermal decomposition at a temperature as low as ≈300 °C. Th
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