Chemistry of the Group VB Pentoxides. VI. The Polymorphism of Nb₂O₅

The polymorphism of Nb2O5 has been reinvestigated. Experimental results indicate the definite existence of two modifications and the possible existence of a third. The δ-modification recently reported has been shown to be a poorly crystallized state of the γ-phase. It is proposed that the α- and β-phases are identical, with the β-modification existing as a two-dimensional array. Transformation temperatures were found to be: amorphous to γ 435°, γ to α 830°. The densities are: amorphous 4.36 g./cm.3 at 25°, γ, 5.17 g./cm.3 at 25° and α, 4.55 g./cm.3 at 25°. None of the transformations was reversible. The α-phase was reindexed on the basis of a monoclinic unit cell with a = 21.34 Å., b = 2.816 Å., c = 19.47 Å. and ≮ β = 120°20′ having 14 molecules/unit cell and an X-ray density of 4.52 g./cm.3. The freezing point of Nb2O5 obtained in the study was 1491°. © 1957, American Chemical Society. All rights reserved.