|Jmol-3D images||Image 1|
|Molar mass||441.893 g/mol|
|Appearance||white, odorless powder|
|Solubility in water||negligible|
|Solubility||insoluble in ethanol, most acid
soluble in hot conc.HF
|Refractive index (nD)||2.275|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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
- 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:
- 2 Ta(OCH2CH3)5 + 5 H2O → Ta2O5 + 10 CH3CH2OH
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.
Tantalum pentoxide is a highly robust material, decomposing thermally above 1470 °C.
- 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.
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.
- Fairbrother, Frederick (1967). The Chemistry of Niobium and Tantalum. New York: Elsevier Publishing Company. pp. 1–28. ISBN 978-0-444-40205-9.
- Nashed, Ramy; Hassan, Walid M. I.; Ismail, Yehea; Allam, Nageh K. (2013). "Unravelling the interplay of crystal structure and electronic band structure of tantalum oxide (Ta2O5)". Physical Chemistry Chemical Physics. doi:10.1039/C2CP43492J.
- Ezhilvalavan, S.; Tseng, T. Y. "Preparation and properties of tantalum pentoxide (Ta2O5) thin films for ultra large scale integrated circuits (ULSIs) application - a review" Journal of Materials Science: Materials in Electronics (1999), 10(1), 9-31.
- Wells, A.F. (1947). Structural Inorganic Chemistry. Oxford: Clarendon Press.
Further reading 
- "Tantalum compounds: ditantalum pentoxide". WebElements: the periodic table on the web. WebElements. Retrieved 18 November 2011.