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High‐Temperature Transmission Electron Microscopy and X‐Ray Powder Diffraction Studies of Polymorphic Phase Transitions in Ba 4 Nb 2 O 9
Author(s) -
Bezjak Jana,
Rečnik Aleksander,
Jančar Boštjan,
Boullay Philippe,
Evans Ivana Radosavljević,
Suvorov Danilo
Publication year - 2009
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2009.03111.x
Subject(s) - transmission electron microscopy , orthorhombic crystal system , materials science , nanocrystalline material , atmospheric temperature range , phase (matter) , crystallography , endothermic process , metastability , x ray crystallography , analytical chemistry (journal) , phase transition , electron diffraction , diffraction , crystal structure , chemistry , nanotechnology , thermodynamics , optics , physics , organic chemistry , adsorption , chromatography
Polymorphic phase transitions in Ba 4 Nb 2 O 9 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‐Ba 4 Nb 2 O 9 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 Ba 4 Nb 2 O 9 becomes moderately sensitive to a loss of BaO. In air the surface of Ba 4 Nb 2 O 9 grains decomposes to nanocrystalline Ba 5 Nb 4 O 15 and BaO, which instantly reacts with atmospheric CO 2 to form BaCO 3 . 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 Ba 3 Nb 2 O 8 phase is observed above 1150°C.