Synthesis, crystal structure and electrical properties of Ba2Nb5-xVxO9 (x<1.9)

The Ba2Nb5-xVxO9 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(l) and Nb(2) sites form the Nb, octahedron, and the Nb(3) site is in the center of the NbO6 octahedron. The Rietveld analysis indicated that V atoms occupied both Nb(l) and Nb(2) sites. The a and c axis lengths decreased with increasing V content (a=4.171-4.106 Angstrom, c=12.2-24-12.136 Angstrom) the substitution range was estimated to be 0less than or equal toxless than or equal to1.9. The electrical resistivity of Ba2Nb5-xVxO9 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 Ba2Nb5-xVxO9 increased with increasing V content and it exhibited positive value at v= 1.5 in 100-300 K while the Seebeck coefficient of Ba2Nb5O9 (x=0) showed a neaative value. (C) 2002 Elsevier Science BV All ri, hts reserved.