Reactivity of T-Nb2O5 or H-Nb2 O5 towards V2O5. Synthesis in the solid state and properties of V4Nb18O55

Abstract

Using the XRD method a comparative study was performed on the reactivity of commercial T-Nb2O5 and prepared from it H-Nb2O5 towards commercial V2O5 as part of the V2O5-Nb2O5 system focusing on the V4Nb18O55, VNbO5 and solid solution of V2O5 in VNb9 O25. It has been shown that V4Nb18 O55 can be obtained as a result of the solid state reaction between the commercial V2O5 and T-Nb2 O5 in the temperature range 600 - 620°C. The parameters of the orthorhombic unit cell for such compound are: a = 0.3972(1) nm, b = 1.7383(4) nm, c = 1.7737(5) nm. The V4Nb18 O55 is stable in the solid state up to 800°C when it starts to melt incongruently with depositing of VNb9 O25. The IR spectrum of V4Nb18 O55 has been compared with the IR spectra of selected niobates of known structures to show structural relations between these compounds. This comparison shows that V4Nb18O55 has crystal structure related to T-Nb2O5, W16Nb18O94 and Ba2 NaNb5O15. On the other hand, reaction between V2O5 and H-Nb2O5 yields a solid solution of V2O5 in VNb9 O25. It has been proposed two models of synthesized solid solution with formulas V1+xNb9-xO25 or V1+xNb9O25+5x/2. Independently of Nb2O5 polymorph, used for synthesis, the metastable compound VNbO5 cannot be synthesized in the solid state below 650°C. © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2009.