Tantalum pentoxide

Tantalum pentoxide
Identifiers
CAS number	1314-61-0 Y
PubChem	518172
Jmol-3D images	Image 1
SMILES O=[Ta](=O)O[Ta](=O)=O
InChI InChI=1S/5O.2Ta
Properties
Molecular formula	Ta2O5
Molar mass	441.893 g/mol
Appearance	white, odorless powder
Density	8.20 g/cm3
Solubility in water	negligible
Solubility	insoluble in ethanol, most acid soluble in hot conc.HF
Refractive index (nD)	2.275
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Tantalum pentoxide, also known as tantalum(V) oxide, is the inorganic compound with the formula Ta₂O₅. It is a white solid that is insoluble in all solvents but is attacked by strong base and hydrofluoric acid. Both orthorhombic and hexagonal phases are known. Ta₂O₅ is a high refractive index, low absorption (i.e. colourless), inert material, which makes it useful for coatings.

Occurrence and preparation

Tantalum occurs in the minerals tantalite and columbite (columbium being an archaic name for niobium), which occur in pegmatites, an igneous rock formation. Mixtures of columbite and tantalite are called coltan. Tantalite was discovered by Anders Gustaf Ekeberg at Ytterby, Sweden, and Kimoto, Finland. The minerals microlite and pyrochlore contain approximately 70% and 10% Ta, respectively.

Tantalum oxide is obtained from the aforementioned ores. The powdered mineral is fused with sodium hydroxide to give a slurry of mixed insoluble "niobic acid" and "tantalic acid", the hydrated oxides. These oxides are extracted with anhydrous hydrofluoric acid and potassium fluoride (or the bifluoride) to yield the salt potassium heptafluorotantalate (K₂TaF₇). This salt can be purified by a liquid-liquid extraction process, using various ketones as the liquids.

(FeMn)(NbTa)₂O₆ + 6 NaOH → Ta₂O₅·7H₂O + Nb₂O₅·7H₂O

½ Ta₂O₅·7H₂O + ½ Nb₂O₅·7H₂O + 14 KF → K₂TaF₇ + K₂NbF₇ + 2 H₂O + 10 KOH

Or,

½ Ta₂O₅ + ½ Nb₂O₅ + 4 KF + 10 HF → K₂TaF₇ + K₂NbF₇ + 5 H₂O

Metallic Ta is obtained by the electrolysis of the purified molten K₂TaF₇. Roasting tantalum in oxygen at temperatures above 1000 °C, gives the pentaoxide:

It can also be produced by hydrolysis of its alkoxide:

2 Ta(OCH₂CH₃)₅ + 5 H₂O → Ta₂O₅ + 10 CH₃CH₂OH

Use

Owing to its high band gap of 3.7 eV, Ta₂O₅ has found a variety of uses in electronics. It is used to make capacitors in automotive electronics, cell phones, and pagers, electronic circuitry; thin-film components; and high-speed tools. In the 1990s, interest grew in the use of tantalum oxide as a high-k dielectric for DRAM capacitor applications. It is used in on-chip MIM capacitors for RF CMOS integrated circuits. Due to its high index of refraction, Ta₂O₅ has been utilized in the fabrication of the glass of many photographic lenses.

Reactions

Tantalum pentoxide is a highly robust material, decomposing thermally above 1470 °C.

Chlorination of Ta₂O₅ with carbon tetrachloride affords tantalum pentachloride:

Ta₂O₅ + 5 CCl₄ → 2 TaCl₅ + 5 COCl₂

Ta₂O₅ can be reduced several different ways, including the use of metallic reductants such as calcium and aluminum.

Ta₂O₅ + 2 Al → Al₂O₃ + TaO₂ + Ta

Ta₂O₅ + Ca → CaO + 2 TaO₂

These reactions can be further modified to yield pure tantalum via stoichiometry, thermal control and electrolysis.

Structure

Both low and high temperature polymorphs exist. The low temperature form is known as β-Ta₂O₅, and the high temperature form is known as α-Ta₂O₅. The transition point between these two forms has been reported as 1360 °C. The transition is slow but reversible. The structures of both polymorphs consist of chains built from octahedral and pentagonal bipyramidal polyhedra sharing opposite vertices. These chains are further joined by sharing edges.

Further reading

• "Tantalum compounds: ditantalum pentoxide". WebElements: the periodic table on the web. WebElements. Retrieved 18 November 2011.