The synthesis and polymorphic phase transitions of Ba4Nb2O9 ceramics

Perovskite-based ceramics within the BaO–(Nb, Ta)2O5 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) (Ba4Nb2O9) were studied at different temperatures Ba4Nb2O9 ceramics were prepared via a conventional solid-state reaction route. Three Ba4Nb2O9 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 Ba5Nb4O15 in all the polymorphs, most abundantly in the α-modification. Superstructure ordering in the α-modification was confirmed by electron diffraction methods. The α-Ba4Nb2O9 was observed to be stable in the temperature range 560–1160 °C; below 560 °C the stable polymorph is β-Ba4Nb2O9, while above 1160 °C the only stable polymorph is γ-Ba4Nb2O9.