Abstract
Perovskite-based ceramics within the BaO–(Nb, Ta) 2O 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 4Nb 2O 9) were studied at different temperatures Ba 4Nb 2O 9 ceramics were prepared via a conventional solid-state reaction route. Three Ba 4Nb 2O 9 polymorphs were isolated. These include a hexagonal (α) and two orthorhombic (γ, β) modifications. Transmission electron microscopy (TEM) revealed an intergranular BaO-rich amorphous phase and nano-crystalline Ba 5Nb 4O 15 in all the polymorphs, most abundantly in the α-modification. Superstructure ordering in the α-modification was confirmed by electron diffraction methods. The α-Ba 4Nb 2O 9 was observed to be stable in the temperature range 560–1160 °C; below 560 °C the stable polymorph is β-Ba 4Nb 2O 9, while above 1160 °C the only stable polymorph is γ-Ba 4Nb 2O 9.
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