Niobium pentoxide

Niobium pentoxide is the inorganic compound with the formula Nb₂O₅. A colorless, insoluble, and fairly unreactive solid, it is the most widespread precursor for other compounds and materials containing niobium. It is predominantly used in alloying, with other specialized applications in capacitors, optical glasses, and the production of lithium niobate.

It has many polymorphic forms all based largely on octahedrally coordinated niobium atoms. The polymorphs are identified with a variety of prefixes. The form most commonly encountered is monoclinic H-Nb₂O₅, which has a complex structure with a unit cell containing 28 niobium atoms and 70 oxygen, where 27 of the niobium atoms are octahedrally coordinated and one tetrahedrally. There is an uncharacterised solid hydrate, Nb₂O₅·nH₂O, the so-called niobic acid (previously called columbic acid), which can be prepared by hydrolysis of a basic solution of niobium pentachloride or Nb₂O₅ dissolved in HF.

Molten niobium pentoxide has lower mean coordination numbers than the crystalline forms, with a structure comprising mostly NbO₅ and NbO₆ polyhedra.

Nb₂O₅ is prepared by hydrolysis of alkali-metal niobates, alkoxides or fluoride using base. Such ostensibly simple procedures afford hydrated oxides that can then be calcined. Pure Nb₂O₅ can also be prepared by hydrolysis of NbCl₅:

A method of production via sol-gel techniques has been reported hydrolysing niobium alkoxides in the presence of acetic acid, followed by calcination of the gels to produce the orthorhombic form, T-Nb₂O₅.

Given that Nb₂O₅ is the most common and robust compound of niobium, many methods, both practical and esoteric, exist for its formation. The oxide for example, arises when niobium metal is oxidised in air. The oxidation of niobium dioxide, NbO₂ in air forms the polymorph, L-Nb₂O₅.

Nano-sized niobium pentoxide particles have been synthesized by LiH reduction of NbCl₅, followed by aerial oxidation as part of a synthesis of nano structured niobates.^{[citation needed]}

Nb₂O₅ is attacked by HF and dissolves in fused alkali.