Reactivity of T-Nb2O5 or H-Nb2O5 towards V2O5. Synthesis in the solid state and properties of V4Nb_(18)O_(55)

摘要

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 V4Nb_(18)O_(55), VNbO5 and solid solution of V2O5 in VNb9O_(25). It has been shown that V4Nb_(18)O_(55) can be obtained as a result of the solid state reaction between the commercial V2O5 and T-Nb2O5 in the temperature range 600 - 620℃. 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 V4Nb_(18)O_(55) is stable in the solid state up to 800℃ when it starts to melt incongruently with depositing of VNb9O_(25). The IR spectrum of V4Nb_(18)O_(55) has been compared with the IR spectra of selected niobates of known structures to show structural relations between these compounds. This comparison shows that V4Nb_(18)O_(55) has crystal structure related to T-Nb2O5, W_(16)Nb_(18)O_(94) and Ba2NaNb5O_(15). On the other hand, reaction between V2O5 and H-Nb2O5 yields a solid solution of V2O5 in VNb9O_(25). It has been proposed two models of synthesized solid solution with formulas V_(1+x)Nb_(9-x)O_(25) or V_(1+x)Nb9O_((25+5x)/2).Independently of Nb2O5 polymorph, used for synthesis, the metastable compound VNbO5 cannot be synthesized in the solid state below 650℃.