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Tantalum pentoxide

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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.[2] It is also extensively used in the production of capacitors, due to its high dielectric constant.

Tantalum pentoxide
Names
IUPAC name
Tantalum(V) oxide
Systematic IUPAC name
Ditantalum pentaoxide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.013.854 Edit this at Wikidata
UNII
  • InChI=1S/5O.2Ta
  • O=[Ta](=O)O[Ta](=O)=O
Properties
Ta2O5
Molar mass 441.893 g/mol
Appearance white, odorless powder
Density β-Ta2O5 = 8.18 g/cm3[1]
α-Ta2O5 = 8.37 g/cm3
Melting point 1,872 °C (3,402 °F; 2,145 K)
negligible
Solubility insoluble in organic solvents and most mineral acids, reacts with HF
Band gap 3.8–5.3 eV
−32.0×10−6 cm3/mol
2.275
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Preparation

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Occurrence

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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

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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[TaF7] + H2[NbOF5] + 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:[3]

H2[TaF7] + 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.[4]

From alkoxides

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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

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The crystal structure of tantalum pentoxide has been the matter of some debate. The bulk material is disordered,[5] 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.[5] 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.[6][7] 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.[8][9]

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).[10]

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.[11]

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.[12][13][14] 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).[15][16][17]

Its dielectric constant is typically about 25[18] although values of over 50 have been reported.[19] In general tantalum pentoxide is considered to be a high-k dielectric material.

Reactions

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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:[20][21]

Ta2O5 + 4 KHF2 + 6 HF → 2 K2[TaF7] + 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
 
Several 10 μF × 30 V DC rated tantalum capacitors, solid-bodied epoxy-dipped type. Polarity is explicitly marked.

Uses

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In electronics

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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.[22][23]

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.[24][25] There are applications of tantalum oxide in resistive switching memories.[26]

In optics

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Due to its high refractive index, Ta2O5 has been utilized in the fabrication of the glass of photographic lenses.[2][27] It can also be deposited as an optical coating with typical applications being antireflection and multilayer filter coatings in near UV to near infrared. [28]

Ta2O5 has also been found to have a high nonlinear refractive index,[29][30] 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[31][32] and Kerr frequency combs[30] 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.[33][34][35]

References

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  1. ^ Reisman, Arnold; Holtzberg, Frederic; Berkenblit, Melvin; Berry, Margaret (20 September 1956). "Reactions of the Group VB Pentoxides with Alkali Oxides and Carbonates. III. Thermal and X-Ray Phase Diagrams of the System K2O or K2CO3 with Ta2O5". Journal of the American Chemical Society. 78 (18): 4514–4520. doi:10.1021/ja01599a003.
  2. ^ a b Fairbrother, Frederick (1967). The Chemistry of Niobium and Tantalum. New York: Elsevier Publishing Company. pp. 1–28. ISBN 978-0-444-40205-9.
  3. ^ Anthony Agulyanski (2004). "Fluorine chemistry in the processing of tantalum and niobium". In Anatoly Agulyanski (ed.). Chemistry of Tantalum and Niobium Fluoride Compounds (1st ed.). Burlington: Elsevier. ISBN 9780080529028.
  4. ^ "Tantite: Tantite mineral information and data". Mindat.org. Retrieved 2016-03-03.
  5. ^ a b Askeljung, Charlotta; Marinder, Bengt-Olov; Sundberg, Margareta (1 November 2003). "Effect of heat treatment on the structure of L-Ta2O5". Journal of Solid State Chemistry. 176 (1): 250–258. Bibcode:2003JSSCh.176..250A. doi:10.1016/j.jssc.2003.07.003.
  6. ^ Stephenson, N. C.; Roth, R. S. (1971). "Structural systematics in the binary system Ta2O5–WO3. V. The structure of the low-temperature form of tantalum oxide L-Ta2O5". Acta Crystallographica Section B. 27 (5): 1037–1044. Bibcode:1971AcCrB..27.1037S. doi:10.1107/S056774087100342X.
  7. ^ Wells, A.F. (1947). Structural Inorganic Chemistry. Oxford: Clarendon Press.
  8. ^ Wolten, G. M.; Chase, A. B. (1 August 1969). "Single-crystal data for β Ta2O5 and A KPO3". Zeitschrift für Kristallographie. 129 (5–6): 365–368. Bibcode:1969ZK....129..365W. doi:10.1524/zkri.1969.129.5-6.365.
  9. ^ Hummel, Hans-U.; Fackler, Richard; Remmert, Peter (1992). "Tantaloxide durch Gasphasenhydrolyse, Druckhydrolyse und Transportreaktion aus 2H-TaS2: Synthesen von TT-Ta2O5 und T-Ta2O5 und Kristallstruktur von T-Ta2O5". Chemische Berichte. 125 (3): 551–556. doi:10.1002/cber.19921250304.
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  24. ^ Wang, X; et al. (2004). "A Novel MONOS-Type Nonvolatile Memory Using High-κ Dielectrics for Improved Data Retention and Programming Speed". IEEE Transactions on Electron Devices. 51 (4): 597–602. Bibcode:2004ITED...51..597W. doi:10.1109/TED.2004.824684.
  25. ^ Zhu, H; et al. (2013). "Design and Fabrication of Ta2O5 Stacks for Discrete Multibit Memory Application". IEEE Transactions on Nanotechnology. 12 (6): 1151–1157. Bibcode:2013ITNan..12.1151Z. doi:10.1109/TNANO.2013.2281817. S2CID 44045227.
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  33. ^ Tong, Amy S. K.; Mitchell, Colin J.; Aghajani, Armen; Sessions, Neil; Senthil Murugan, G.; Mackenzie, Jacob I.; Wilkinson, James S. (2020-09-01). "Spectroscopy of thulium-doped tantalum pentoxide waveguides on silicon". Optical Materials Express. 10 (9): 2201. Bibcode:2020OMExp..10.2201T. doi:10.1364/OME.397011. ISSN 2159-3930.
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