The synthesis and polymorphic phase transitions of Ba4Nb2O9 ceramics

✍ Scribed by Jana Bezjak; Boštjan Jančar; Aleksander Rečnik; Danilo Suvorov

• Publisher: Elsevier Science
• Year: 2008
• Tongue: English
• Weight: 646 KB
• Volume: 28
• Category: Article
• ISSN: 0955-2219
• DOI: 10.1016/j.jeurceramsoc.2008.04.003

✦ Synopsis

Perovskite-based ceramics within the BaO-(Nb, Ta) 2 O 5 system possess excellent microwave dielectric properties, which make them suitable for applications in communication systems. The polymorphic phase transitions and the stability of the individual polymorphs of tetra-barium di-niobate (V) (Ba 4 Nb 2 O 9 ) were studied at different temperatures Ba 4 Nb 2 O 9 ceramics were prepared via a conventional solid-state reaction route. Three Ba 4 Nb 2 O 9 polymorphs were isolated. These include a hexagonal (␣) and two orthorhombic (␥, ␤) modifications. Transmission electron microscopy (TEM) revealed an intergranular BaO-rich amorphous phase and nano-crystalline Ba 5 Nb 4 O 15 in all the polymorphs, most abundantly in the ␣-modification. Superstructure ordering in the ␣-modification was confirmed by electron diffraction methods. The ␣-Ba 4 Nb 2 O 9 was observed to be stable in the temperature range 560-1160 • C; below 560 • C the stable polymorph is ␤-Ba 4 Nb 2 O 9 , while above 1160 • C the only stable polymorph is ␥-Ba 4 Nb 2 O 9 .