High-Temperature Transmission Electron Microscopy and X-Ray Powder Diffraction Studies of Polymorphic Phase Transitions in Ba4Nb2O9 |
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Authors: | Jana Bezjak Aleksander Renik Botjan Janar Philippe Boullay Ivana Radosavljevi Evans Danilo Suvorov |
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Affiliation: | Advanced Materials Department, Jo?ef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia; Nanostructured Materials Department, Jo?ef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia; Laboratoire CRISMAT UMR CNRS 6508, 14050 Caen Cedex 4, France; Department of Chemistry, Durham University, Durham DH1 3LE, U.K. |
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Abstract: | Polymorphic phase transitions in Ba4Nb2O9 were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba4Nb2O9 results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ~490°C Ba4Nb2O9 becomes moderately sensitive to a loss of BaO. In air the surface of Ba4Nb2O9 grains decomposes to nanocrystalline Ba5Nb4O15 and BaO, which instantly reacts with atmospheric CO2 to form BaCO3. Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba3Nb2O8 phase is observed above 1150°C. |
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