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Tantalum pentoxide

Tantalum pentoxide, also known astantalum(V) oxide, is theinorganic compound with theformulaTa
2O
5. It is a white solid that is insoluble in all solvents but is attacked by strong bases andhydrofluoric acid.Ta
2O
5 is an inert material with a highrefractive index and low absorption (i.e. colourless), which makes it useful for coatings.[2] It is also extensively used in the production ofcapacitors, due to its highdielectric constant.

Tantalum pentoxide
Names
IUPAC name
Tantalum(V) oxide
Systematic IUPAC name
Ditantalum pentaoxide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard100.013.854Edit this at Wikidata
UNII
  • InChI=1S/5O.2Ta
  • O=[Ta](=O)O[Ta](=O)=O
Properties
Ta2O5
Molar mass441.893 g/mol
Appearancewhite, odorless powder
Densityβ-Ta2O5 = 8.18 g/cm3[1]
α-Ta2O5 = 8.37 g/cm3
Melting point1,872 °C (3,402 °F; 2,145 K)
negligible
Solubilityinsoluble in organic solvents and mostmineral acids, reacts with HF
Band gap3.8–5.3 eV
−32.0×10−6 cm3/mol
2.275
Except where otherwise noted, data are given for materials in theirstandard 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 mineralstantalite andcolumbite (columbium being an archaic name for niobium), which occur inpegmatites, an igneous rock formation. Mixtures of columbite and tantalite are calledcoltan. Tantalum was discovered in Tantalite in 1802 byAnders Gustaf Ekeberg atYtterby, Sweden, and Kimoto, Finland. The mineralsmicrolite andpyrochlore contain approximately 70% and 10% Ta, respectively.

Refining

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Tantalum ores often contain significant amounts ofniobium, which is itself a valuable metal. As such, both metals are extracted so that they may be sold. The overall process is one ofhydrometallurgy and begins with aleaching step; in which the ore is treated withhydrofluoric acid andsulfuric acid to produce water-solublehydrogen fluorides, such as theheptafluorotantalate. This allows the metals to be separated from the various non-metallic impurities in the rock.

(FeMn)(NbTa)2O6 + 16HF → H2[TaF7] + H2[NbOF5] +FeF2 +MnF2 + 6 H2O

The tantalum and niobium hydrogenfluorides are then removed from theaqueous solution byliquid-liquid extraction usingorganic solvents, such ascyclohexanone ormethyl 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 offluorides. Separation of the tantalum and niobium is then achieved bypH 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 aqueousammonia to givehydrated tantalum oxide (Ta2O5(H2O)x), which iscalcinated to tantalum pentoxide (Ta2O5) as described in these idealized equations:[3]

H2[TaF7] + 5 H2O + 7NH3 →⁠1/2⁠ Ta2O5(H2O)5 + 7NH4F
Ta2O5(H2O)5 → Ta2O5 + 5 H2O

Natural pure tantalum oxide is known as the mineraltantite, although it is exceedingly rare.[4]

From alkoxides

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Tantalum oxide is frequently used in electronics, often in the form ofthin films. For these applications it can be produced byMOCVD (or related techniques), which involves thehydrolysis of its volatilehalides oralkoxides:

Ta2(OEt)10 + 5 H2O → Ta2O5 + 10 EtOH
2TaCl5 + 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 isdisordered,[5] being eitheramorphous orpolycrystalline; withsingle crystals being difficult to grow. As suchXray crystallography has largely been limited topowder diffraction, which provides less structural information.

At least 2polymorphs 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 isorthorhombic in both cases, with thespace group of β-Ta2O5 being identified asPna2 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 amonoclinic 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 aninsulator and itsband 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 moreamorphous the material the greater its observed band gap.These observed values are significantly higher than those predicted bycomputational chemistry (2.3 - 3.8 eV).[15][16][17]

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

Reactions

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Ta2O5 does not react appreciably with either HCl or HBr, however it will dissolve inhydrofluoric acid, and reacts withpotassium bifluoride and HF according to the following equation:[20][21]

Ta2O5 + 4 KHF2 + 6 HF → 2K2[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 + 5CaO
 
Several10 μF × 30 V DC ratedtantalum capacitors, solid-bodied epoxy-dipped type. Polarity is explicitly marked.

Uses

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In electronics

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Owing to its highband gap anddielectric constant, tantalum pentoxide has found a variety of uses in electronics, particularly intantalum capacitors. These are used inautomotive 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 ahigh-k dielectric forDRAM capacitor applications.[22][23]

It is used in on-chip metal-insulator-metal capacitors for high frequencyCMOS integrated circuits. Tantalum oxide may have applications as the charge trapping layer fornon-volatile memories.[24][25] There are applications of tantalum oxide inresistive switching memories.[26]

In optics

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Due to its highrefractive index, Ta2O5 has been utilized in the fabrication of theglass ofphotographic lenses.[2][27] It can also be deposited as anoptical coating with typical applications being antireflection and multilayer filter coatings in nearUV to nearinfrared.[28]

Ta2O5 has also been found to have a high nonlinearrefractive index,[29][30] on the order of three times that ofsilicon nitiride, which has led to interest in the utilization of Ta2O5 inphotonic integrated circuits. Ta2O5 has been recently utilized as the material platform for the generation ofsupercontinuum[31][32] andKerr frequency combs[30] inwaveguides andoptical ring resonators. Through the addition ofrare-earth dopants in the deposition process, Ta2O5 waveguide lasers have been presented for a variety of applications, such as remote sensing andLiDAR.[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. ^abFairbrother, 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. Retrieved2016-03-03.
  5. ^abAskeljung, 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.
  10. ^Zibrov, I. P.; Filonenko, V. P.; Sundberg, M.; Werner, P.-E. (1 August 2000). "Structures and phase transitions of B-Ta2O5 and Z-Ta2O5: two high-pressure forms of Ta2O5".Acta Crystallographica Section B.56 (4):659–665.doi:10.1107/S0108768100005462.PMID 10944257.S2CID 22330435.
  11. ^Alderman, O. L. G.; Benmore, C.J.; Neuefeind, J.; Coillet, E.; Mermet, A.; Martinez, V.; Tamalonis, A.; Weber, R. (2018)."Amorphous tantala and its relationship with the molten state".Physical Review Materials.2 (4): 043602.Bibcode:2018PhRvM...2d3602A.doi:10.1103/PhysRevMaterials.2.043602.
  12. ^Kukli, Kaupo; Aarik, Jaan; Aidla, Aleks; Kohan, Oksana; Uustare, Teet; Sammelselg, Väino (1995). "Properties of tantalum oxide thin films grown by atomic layer deposition".Thin Solid Films.260 (2):135–142.Bibcode:1995TSF...260..135K.doi:10.1016/0040-6090(94)06388-5.
  13. ^Fleming, R. M.; Lang, D. V.; Jones, C. D. W.; Steigerwald, M. L.; Murphy, D. W.; Alers, G. B.; Wong, Y.-H.; van Dover, R. B.; Kwo, J. R.; Sergent, A. M. (1 January 2000). "Defect dominated charge transport in amorphous Ta2O5 thin films".Journal of Applied Physics.88 (2): 850.Bibcode:2000JAP....88..850F.doi:10.1063/1.373747.
  14. ^Murawala, Prakash A.; Sawai, Mikio; Tatsuta, Toshiaki; Tsuji, Osamu; Fujita, Shizuo; Fujita, Shigeo (1993). "Structural and Electrical Properties of Ta2O5 Grown by the Plasma-Enhanced Liquid Source CVD Using Penta Ethoxy Tantalum Source".Japanese Journal of Applied Physics.32 (Part 1, No. 1B):368–375.Bibcode:1993JaJAP..32..368M.doi:10.1143/JJAP.32.368.S2CID 97813703.
  15. ^Ramprasad, R. (1 January 2003). "First principles study of oxygen vacancy defects in tantalum pentoxide".Journal of Applied Physics.94 (9):5609–5612.Bibcode:2003JAP....94.5609R.doi:10.1063/1.1615700.
  16. ^Sawada, H.; Kawakami, K. (1 January 1999). "Electronic structure of oxygen vacancy in Ta2O5".Journal of Applied Physics.86 (2): 956.Bibcode:1999JAP....86..956S.doi:10.1063/1.370831.
  17. ^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.15 (5):1352–7.Bibcode:2013PCCP...15.1352N.doi:10.1039/C2CP43492J.PMID 23243661.
  18. ^Macagno, V.; Schultze, J.W. (1 December 1984). "The growth and properties of thin oxide layers on tantalum electrodes".Journal of Electroanalytical Chemistry and Interfacial Electrochemistry.180 (1–2):157–170.doi:10.1016/0368-1874(84)83577-7.
  19. ^Hiratani, M.; Kimura, S.; Hamada, T.; Iijima, S.; Nakanishi, N. (1 January 2002). "Hexagonal polymorph of tantalum–pentoxide with enhanced dielectric constant".Applied Physics Letters.81 (13): 2433.Bibcode:2002ApPhL..81.2433H.doi:10.1063/1.1509861.
  20. ^Agulyansky, A (2003). "Potassium fluorotantalate in solid, dissolved and molten conditions".J. Fluorine Chem.123 (2):155–161.doi:10.1016/S0022-1139(03)00190-8.
  21. ^Brauer, Georg (1965).Handbook of preparative inorganic chemistry. Academic Press. p. 256.ISBN 978-0-12-395591-3.
  22. ^Ezhilvalavan, S.; Tseng, T. Y. (1999). "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.10 (1):9–31.doi:10.1023/A:1008970922635.S2CID 55644772.
  23. ^Chaneliere, C; Autran, J L; Devine, R A B; Balland, B (1998). "Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications".Materials Science and Engineering: R.22 (6):269–322.doi:10.1016/S0927-796X(97)00023-5.
  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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