Tantalum pentoxide, also known as
tantalum(V) oxide, is the
inorganic compound with the
formulaTa 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.
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. Tantalite was discovered by
Anders Gustaf Ekeberg[when?] 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.
The tantalum and niobium hydrogenflorides 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]
Natural pure tantalum oxide is known as the mineral
tantite, although it is exceedingly rare.[4]
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:
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]
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).[9]
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.[10]
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.[11][12][13] 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).[14][15][16]
Its
dielectric constant is typically about 25[17] although values of over 50 have been reported.[18] 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:[19][20]
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.[21][22]
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.[23][24] There are applications of tantalum oxide in
resistive switching memories.[25]
^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.
^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.
ISBN9780080529028.
^
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.
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^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:
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doi:
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^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.
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^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.
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^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.
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^Ramprasad, R. (1 January 2003). "First principles study of oxygen vacancy defects in tantalum pentoxide". Journal of Applied Physics. 94 (9): 5609–5612.
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^Sawada, H.; Kawakami, K. (1 January 1999). "Electronic structure of oxygen vacancy in Ta2O5". Journal of Applied Physics. 86 (2): 956.
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^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.
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^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.
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^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.
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