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Tantalum pentoxide

Names
IUPAC name Tantalum(V) oxide
Systematic IUPAC name Ditantalum pentaoxide
Identifiers
CAS Number	1314-61-0
3D model (JSmol)	Interactive image
ChemSpider	452513
ECHA InfoCard	100.013.854
PubChem CID	518712
UNII	OEZ64Z53M4
CompTox Dashboard (EPA)	DTXSID10893849
InChI InChI=1S/5O.2Ta
SMILES O=(=O)O(=O)=O
Properties
Chemical formula	Ta2O5
Molar mass	441.893 g/mol
Appearance	white, odorless powder
Density	β-Ta2O5 = 8.18 g/cm α-Ta2O5 = 8.37 g/cm
Melting point	1,872 °C (3,402 °F; 2,145 K)
Solubility in water	negligible
Solubility	insoluble in organic solvents and most mineral acids, reacts with HF
Band gap	3.8–5.3 eV
Magnetic susceptibility (χ)	−32.0×10 cm/mol
Refractive index (nD)	2.275
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). N verify (what is  ?) Infobox references

Tantalum pentoxide, also known as tantalum(V) oxide, is the inorganic compound with the formula Ta
₂O
₅. It is a white solid that is insoluble in all solvents but is attacked by strong bases and hydrofluoric acid. Ta
₂O
₅ is an inert material with a high refractive index and low absorption (i.e. colourless), which makes it useful for coatings. It is also extensively used in the production of capacitors, due to its high dielectric constant.

Preparation

Occurrence

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. Tantalum was discovered in Tantalite in 1802 by Anders Gustaf Ekeberg at Ytterby, Sweden, and Kimoto, Finland. The minerals microlite and pyrochlore contain approximately 70% and 10% Ta, respectively.

Refining

Tantalum ores often contain significant amounts of niobium, which is itself a valuable metal. As such, both metals are extracted so that they may be sold. The overall process is one of hydrometallurgy and begins with a leaching step; in which the ore is treated with hydrofluoric acid and sulfuric acid to produce water-soluble hydrogen fluorides, such as the heptafluorotantalate. This allows the metals to be separated from the various non-metallic impurities in the rock.

(FeMn)(NbTa)₂O₆ + 16 HF → H₂ + H₂ + FeF₂ + MnF₂ + 6 H₂O

The tantalum and niobium hydrogenfluorides are then removed from the aqueous solution by liquid-liquid extraction using organic solvents, such as cyclohexanone or methyl isobutyl ketone. This step allows the simple removal of various metal impurities (e.g. iron and manganese) which remain in the aqueous phase in the form of fluorides. Separation of the tantalum and niobium is then achieved by pH adjustment. Niobium requires a higher level of acidity to remain soluble in the organic phase and can hence be selectively removed by extraction into less acidic water. The pure tantalum hydrogen fluoride solution is then neutralised with aqueous ammonia to give hydrated tantalum oxide (Ta₂O₅(H₂O)_x), which is calcinated to tantalum pentoxide (Ta₂O₅) as described in these idealized equations:

H₂ + 5 H₂O + 7 NH₃ → ⁠1/2⁠ Ta₂O₅(H₂O)₅ + 7 NH₄F

Ta₂O₅(H₂O)₅ → Ta₂O₅ + 5 H₂O

Natural pure tantalum oxide is known as the mineral tantite, although it is exceedingly rare.

From alkoxides

Tantalum oxide is frequently used in electronics, often in the form of thin films. For these applications it can be produced by MOCVD (or related techniques), which involves the hydrolysis of its volatile halides or alkoxides:

Ta₂(OEt)₁₀ + 5 H₂O → Ta₂O₅ + 10 EtOH

2 TaCl₅ + 5 H₂O → Ta₂O₅ + 10 HCl

Structure and properties

The crystal structure of tantalum pentoxide has been the matter of some debate. The bulk material is disordered, being either amorphous or polycrystalline; with single crystals being difficult to grow. As such Xray crystallography has largely been limited to powder diffraction, which provides less structural information.

At least 2 polymorphs are known to exist. A low temperature form, known as L- or β-Ta₂O₅, and the high temperature form known as H- or α-Ta₂O₅. The transition between these two forms is slow and reversible; taking place between 1000 and 1360 °C, with a mixture of structures existing at intermediate temperatures. The structures of both polymorphs consist of chains built from octahedral TaO₆ and pentagonal bipyramidal TaO₇ polyhedra sharing opposite vertices; which are further joined by edge-sharing. The overall crystal system is orthorhombic in both cases, with the space group of β-Ta₂O₅ being identified as Pna2 by single crystal X-ray diffraction.

A high pressure form (Z-Ta₂O₅) has also been reported, in which the Ta atoms adopt a 7 coordinate geometry to give a monoclinic structure (space group C2).

Purely amorphous tantalum pentoxide has a similar local structure to the crystalline polymorphs, built from TaO₆ and TaO₇ polyhedra, while the molten liquid phase has a distinct structure based on lower coordination polyhedra, mainly TaO₅ and TaO₆.

The difficulty in forming material with a uniform structure has led to variations in its reported properties. Like many metal oxides Ta₂O₅ is an insulator and its band gap has variously been reported as being between 3.8 and 5.3 eV, depending on the method of manufacture. In general the more amorphous the material the greater its observed band gap. These observed values are significantly higher than those predicted by computational chemistry (2.3 - 3.8 eV).

Its dielectric constant is typically about 25 although values of over 50 have been reported. In general tantalum pentoxide is considered to be a high-k dielectric material.

Reactions

Ta₂O₅ does not react appreciably with either HCl or HBr, however it will dissolve in hydrofluoric acid, and reacts with potassium bifluoride and HF according to the following equation:

Ta₂O₅ + 4 KHF₂ + 6 HF → 2 K₂ + 5 H₂O

Ta₂O₅ can be reduced to metallic Ta via the use of metallic reductants such as calcium and aluminium.

Ta₂O₅ + 5 Ca → 2 Ta + 5 CaO

Uses

In electronics

Owing to its high band gap and dielectric constant, tantalum pentoxide has found a variety of uses in electronics, particularly in tantalum capacitors. These are used 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 metal-insulator-metal capacitors for high frequency CMOS integrated circuits. Tantalum oxide may have applications as the charge trapping layer for non-volatile memories. There are applications of tantalum oxide in resistive switching memories.

In optics

Due to its high refractive index, Ta₂O₅ has been utilized in the fabrication of the glass of photographic lenses. It can also be deposited as an optical coating with typical applications being antireflection and multilayer filter coatings in near UV to near infrared.

Ta₂O₅ has also been found to have a high nonlinear refractive index, on the order of three times that of silicon nitiride, which has led to interest in the utilization of Ta₂O₅ in photonic integrated circuits. Ta₂O₅ has been recently utilized as the material platform for the generation of supercontinuum and Kerr frequency combs in waveguides and optical ring resonators. Through the addition of rare-earth dopants in the deposition process, Ta₂O₅ waveguide lasers have been presented for a variety of applications, such as remote sensing and LiDAR.

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• Tantalum(V) compounds
• High-κ dielectrics
• Transition metal oxides