# Niobium pentoxide

Niobium pentoxide is the inorganic compound with the formula Nb$_2$O$_5$. 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.

## Structure

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-Nb$_2$O$_5$, 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, Nb$_2$O$_5$·nH$_2$O, the so-called niobic acid (previously called columbic acid), which can be prepared by hydrolysis of a basic solution of niobium pentachloride or Nb$_2$O$_5$ dissolved in HF.
Molten niobium pentoxide has lower mean coordination numbers than the crystalline forms, with a structure comprising mostly NbO$_5$ and NbO$_6$ polyhedra.

## Production
### Hydrolysis

Nb$_2$O$_5$ 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 Nb$_2$O$_5$ can also be prepared by hydrolysis of NbCl$_5$:
    2 NbCl$_5$ + 5 H$_2$O → Nb$_2$O$_5$ + 10 HCl
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-Nb$_2$O$_5$.

### Oxidation

Given that Nb$_2$O$_5$ 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, NbO$_2$ in air forms the polymorph, L-Nb$_2$O$_5$.
Nano-sized niobium pentoxide particles have been synthesized by LiH reduction of NbCl$_5$, followed by aerial oxidation as part of a synthesis of nano structured niobates.

## Reactions

Nb$_2$O$_5$ is attacked by HF and dissolves in fused alkali.

### Reduction to the metal

The conversion of Nb$_2$O$_5$ is the main route for the industrial production of niobium metal. In the 1980s, about 15,000,000 kg of Nb$_2$O$_5$ were consumed annually for reduction to the metal. The main method is reduction of this oxide with aluminium:
    3 Nb$_2$O$_5$ + 10 Al → 6 Nb + 5 Al$_2$O$_3$
An alternative but less practiced route involves carbothermal reduction, which proceeds via reduction with carbon and forms the basis of the two stage Balke process:
    Nb$_2$O$_5$ + 7 C → 2 NbC + 5 CO (heated under vacuum at 1800 °C)
    5 NbC + Nb$_2$O$_5$ → 7 Nb + 5 CO

### Conversion to halides

Many methods are known for conversion of Nb$_2$O$_5$ to the halides. The main problem is incomplete reaction to give the oxyhalides. In the laboratory, the conversion can be effected with thionyl chloride:
    Nb$_2$O$_5$ + 5 SOCl$_2$ → 2 NbCl$_5$ + 5 SO$_2$
Nb$_2$O$_5$ reacts with CCl$_4$ to give niobium oxychloride NbOCl$_3$.

### Conversion to niobates

Treating Nb$_2$O$_5$ with aqueous NaOH at 200 °C can give crystalline sodium niobate, NaNbO$_3$ whereas the reaction with KOH may yield soluble Lindqvist-type hexaniobates, Nb $_6$O$^{8−}$ $_{19}$. Lithium niobates such as LiNbO$_3$ and Li$_3$NbO$_4$ can be prepared by reaction lithium carbonate and Nb$_2$O$_5$.

### Conversion to reduced niobium oxides

High temperature reduction with H$_2$ gives NbO$_2$:
    Nb$_2$O$_5$ + H$_2$ → 2 NbO$_2$ + H$_2$O
Niobium monoxide arises from a comproportionation using an arc-furnace:
    Nb$_2$O$_5$ + 3Nb → 5 NbO
The burgundy-coloured niobium(III) oxide, one of the first superconducting oxides, can be prepared again by an comproportionation:
    Li$_3$NbO$_4$ + 2 NbO → 3 LiNbO$_2$

## Uses

Niobium pentoxide is used mainly in the production of niobium metal, but specialized applications exist in the production of optical glasses and lithium niobate.
Thin films of Nb$_2$O$_5$ form the dielectric layers in niobium electrolytic capacitors.
Nb$_2$O$_5$ have been considered for use as an anode in a lithium-ion battery, given that their ordered crystalline structure allows charging speeds of 225 mAh g$^{−1}$ at 200 mA g$^{−1}$ across 400 cycles, at a Coulombic efficiency of 99.93%.