Synthesis, crystal structure and electrical properties of Ba₂Nb_5-xV_xO₉ (x<1.9)

The Ba₂Nb_5-xV_xO₉ solid solution was synthesized by replacement of Nb by V in Ba₂Nb₅O₉. The polycrystalline samples were prepared by solid state reaction at 1773 K under flowing Ar. Ba₂Nb₅O₉ has three Nb sites; the Nb(1) and Nb(2) sites form the Nb₆ octahedron, and the Nb(3) site is in the center of the NbO₆ octahedron. The Rietveld analysis indicated that V atoms occupied both Nb(1) and Nb(2) sites. The a and c axis lengths decreased with increasing V content (a=4.171-4.106 Å, c=12.224-12.136 Å), the substitution range was estimated to be 0≤x<1.9. The electrical resistivity of Ba₂Nb_5-xV_xO₉ showed metallic behavior at 100-350 K, which increased with increasing V content. Substitution of V atoms changed the main conductive carrier; the Seebeck coefficient of Ba₂Nb_5-xV_xO₉ increased with increasing V content and it exhibited positive value at x=1.5 in 100-300 K while the Seebeck coefficient of Ba₂Nb₅O₉ (x=0) showed a negative value.