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Tantalum pentoxide
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<h2 id="preparation">Preparation</h2> <h3>Occurrence</h3> <p>Tantalum occurs in the minerals <a href="/facts/Tantalite/oEVgGTLG">tantalite</a> and <a href="/facts/Columbite/TLtIHgpR">columbite</a> (columbium being an archaic name for niobium), which occur in <a href="/facts/Pegmatite/qW5BUXPg">pegmatites</a>, an igneous rock formation. Mixtures of columbite and tantalite are called <a href="/facts/Coltan/5s1wmyfr">coltan</a>. Tantalum was discovered in Tantalite in 1802 by <a href="/facts/Anders_Gustaf_Ekeberg/pFLBthwv">Anders Gustaf Ekeberg</a> at <a href="/facts/Ytterby/6CaEoUjc">Ytterby</a>, Sweden, and Kimoto, Finland. The minerals <a href="/facts/Microlite/qqQwCjcV">microlite</a> and <a href="/facts/Pyrochlore/jxGjaSv8">pyrochlore</a> contain approximately 70% and 10% Ta, respectively. </p> <h3>Refining</h3> <p>Tantalum ores often contain significant amounts of <a href="/facts/Niobium/3zNI8AKp">niobium</a>, which is itself a valuable metal. As such, both metals are extracted so that they may be sold. The overall process is one of <a href="/facts/Hydrometallurgy/RUW4cffs">hydrometallurgy</a> and begins with a <a href="/facts/Leaching_(metallurgy)/lkF9x75r">leaching</a> step; in which the ore is treated with <a href="/facts/Hydrofluoric_acid/wdXqu4bE">hydrofluoric acid</a> and <a href="/facts/Sulfuric_acid/Wom6fymJ">sulfuric acid</a> to produce water-soluble <a href="/facts/Hydrogen_fluoride/XfQxlqR6">hydrogen fluorides</a>, such as the <a href="/facts/Potassium_heptafluorotantalate/yup3rQw5">heptafluorotantalate</a>. This allows the metals to be separated from the various non-metallic impurities in the rock. </p> (FeMn)(NbTa)2O6 + 16 <a href="/facts/Hydrofluoric_acid/wdXqu4bE">HF</a> → H2[TaF7] + H2[NbOF5] + <a href="/facts/Iron(II)_fluoride/M69T1Xkl">FeF2</a> + <a href="/facts/Manganese(II)_fluoride/bFDgHv7z">MnF2</a> + 6 H2O <p>The tantalum and niobium hydrogenfluorides are then removed from the <a href="/facts/Aqueous/eRNr3MH2">aqueous</a> solution by <a href="/facts/Liquid%E2%80%93liquid_extraction/qVfck5T1">liquid-liquid extraction</a> using <a href="/facts/Organic_solvents/LoMPMz8S">organic solvents</a>, such as <a href="/facts/Cyclohexanone/Hf3YTWjB">cyclohexanone</a> or <a href="/facts/Methyl_isobutyl_ketone/jjmynEPo">methyl isobutyl ketone</a>. 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 <a href="/facts/Fluoride/LpqNMzwa">fluorides</a>. Separation of the tantalum and niobium is then achieved by <a href="/facts/PH/RnSv8D7y">pH</a> 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 <a href="/facts/Ammonia/MtLtJFtz">ammonia</a> to give <a href="/facts/Water_of_hydration/sbaKJNJX">hydrated</a> tantalum oxide (Ta2O5(H2O)x), which is <a href="/facts/Calcination/hghqlHwo">calcinated</a> to tantalum pentoxide (Ta2O5) as described in these idealized equations:<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> </p> H2[TaF7] + 5 H2O + 7 <a href="/facts/Ammonia/MtLtJFtz">NH3</a> → 1/2 Ta2O5(H2O)5 + 7 <a href="/facts/Ammonium_fluoride/uVcvFh2b">NH4F</a> Ta2O5(H2O)5 → Ta2O5 + 5 H2O <p>Natural pure tantalum oxide is known as the mineral <a href="/facts/Tantite/j9yHwbFB">tantite</a>, although it is exceedingly rare.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p> <h3>From alkoxides</h3> <p>Tantalum oxide is frequently used in electronics, often in the form of <a href="/facts/Thin_film/nSeoUT4Q">thin films</a>. For these applications it can be produced by <a href="/facts/MOCVD/Ldg0BICo">MOCVD</a> (or related techniques), which involves the <a href="/facts/Hydrolysis/JTvwIHPs">hydrolysis</a> of its volatile <a href="/facts/Halide/ujYu8kGD">halides</a> or <a href="/facts/Alkoxide/RqG0rm1v">alkoxides</a>: </p> <a href="/facts/Tantalum(V)_ethoxide/8fYTSNh1">Ta2(OEt)10</a> + 5 H2O → Ta2O5 + 10 EtOH 2 <a href="/facts/Tantalum_chloride/AMxt1Iyq">TaCl5</a> + 5 H2O → Ta2O5 + 10 HCl <h2 id="structure-and-properties">Structure and properties</h2> <p>The crystal structure of tantalum pentoxide has been the matter of some debate. The bulk material is <a href="/facts/Disordered_phase/VAqeaphv">disordered</a>,<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> being either <a href="/facts/Amorphous/YTIzmrR0">amorphous</a> or <a href="/facts/Polycrystalline/nXSgR0wp">polycrystalline</a>; with <a href="/facts/Single_crystal/UMG5pFp9">single crystals</a> being difficult to grow. As such <a href="/facts/Xray_crystallography/WtI0F5DN">Xray crystallography</a> has largely been limited to <a href="/facts/Powder_diffraction/y5uDg49u">powder diffraction</a>, which provides less structural information. </p><p>At least 2 <a href="/facts/Polymorphism_(materials_science)/bu2tqsB9">polymorphs</a> 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.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> 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.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> The overall crystal system is <a href="/facts/Orthorhombic/3zo4F8P0">orthorhombic</a> in both cases, with the <a href="/facts/Space_group/2XQx0J7M">space group</a> of β-Ta2O5 being identified as <i>Pna2</i> by single crystal X-ray diffraction.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p><p>A high pressure form (<i>Z</i>-Ta2O5) has also been reported, in which the Ta atoms adopt a 7 coordinate geometry to give a <a href="/facts/Monoclinic/5PCwXvzX">monoclinic</a> structure (space group C2).<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p><p>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.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>The difficulty in forming material with a uniform structure has led to variations in its reported properties. Like many metal oxides Ta2O5 is an <a href="/facts/Insulator_(electricity)/SfSXkrh5">insulator</a> and its <a href="/facts/Band_gap/IjIeh95l">band gap</a> has variously been reported as being between 3.8 and 5.3 eV, depending on the method of manufacture.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a><a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a><a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> In general the more <a href="/facts/Amorphous/YTIzmrR0">amorphous</a> the material the greater its observed band gap. These observed values are significantly higher than those predicted by <a href="/facts/Computational_chemistry/KyuCgRV3">computational chemistry</a> (2.3 - 3.8 eV).<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a><a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> </p><p>Its <a href="/facts/Dielectric_constant/vnKbPTn1">dielectric constant</a> is typically about 25<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> although values of over 50 have been reported.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> In general tantalum pentoxide is considered to be a <a href="/facts/High-k_dielectric/4E8qtXmp">high-k dielectric</a> material. </p> <h2 id="reactions">Reactions</h2> <p>Ta2O5 does not react appreciably with either HCl or HBr, however it will dissolve in <a href="/facts/Hydrofluoric_acid/wdXqu4bE">hydrofluoric acid</a>, and reacts with <a href="/facts/Potassium_bifluoride/MVGNmlMg">potassium bifluoride</a> and HF according to the following equation:<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a><a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> </p> Ta2O5 + 4 KHF2 + 6 HF → 2 <a href="/facts/Potassium_heptafluorotantalate/yup3rQw5">K2[TaF7] </a> + 5 H2O <p>Ta2O5 can be reduced to metallic Ta via the use of metallic reductants such as calcium and aluminium. </p> Ta2O5 + 5 Ca → 2 Ta + 5 <a href="/facts/Calcium_oxide/Wh9C8l7j">CaO</a> <h2 id="uses">Uses</h2> <h3>In electronics</h3> <p>Owing to its high <a href="/facts/Band_gap/IjIeh95l">band gap</a> and <a href="/facts/Dielectric_constant/vnKbPTn1">dielectric constant</a>, tantalum pentoxide has found a variety of uses in electronics, particularly in <a href="/facts/Tantalum_capacitor/YksF94PD">tantalum capacitors</a>. These are used in <a href="/facts/Automotive_electronics/CtTEQk9e">automotive electronics</a>, 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 <a href="/facts/High-k_dielectric/4E8qtXmp">high-k dielectric</a> for <a href="/facts/DRAM/koI9FGo5">DRAM</a> capacitor applications.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a><a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> </p><p>It is used in on-chip metal-insulator-metal capacitors for high frequency <a href="/facts/CMOS/NJIWYgBl">CMOS</a> integrated circuits. Tantalum oxide may have applications as the charge trapping layer for <a href="/facts/Non-volatile_memory/EN72wMD9">non-volatile memories</a>.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a><a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> There are applications of tantalum oxide in <a href="/facts/Resistive_random-access_memory/5gjWEHzQ">resistive switching memories</a>.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> </p> <h3>In optics</h3> <p>Due to its high <a href="/facts/Refractive_index/yjkzvgeE">refractive index</a>, Ta2O5 has been utilized in the fabrication of the <a href="/facts/Glass/hezRsAGf">glass</a> of <a href="/facts/Photographic_lens/TkL6sd1b">photographic lenses</a>.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a><a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> It can also be deposited as an <a href="/facts/Optical_coating/pGnrNRn6">optical coating</a> with typical applications being antireflection and multilayer filter coatings in near <a href="/facts/Ultraviolet/kncBUqn5">UV</a> to near <a href="/facts/Infrared/44XdJNcc">infrared</a>. <a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> </p><p>Ta2O5 has also been found to have a high nonlinear <a href="/facts/Refractive_index/yjkzvgeE">refractive index</a>,<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a><a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> on the order of three times that of <a href="/facts/Silicon_nitride/58X9QPQJ">silicon nitiride</a>, which has led to interest in the utilization of Ta2O5 in <a href="/facts/Photonic_integrated_circuit/C7skzpMo">photonic integrated circuits</a>. Ta2O5 has been recently utilized as the material platform for the generation of <a href="/facts/Supercontinuum/1NrW5nMY">supercontinuum</a><a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a><a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> and <a href="/facts/Kerr_frequency_comb/L51VNzQw">Kerr frequency combs</a><a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> in <a href="/facts/Waveguide_(optics)/CQ6AnRAl">waveguides</a> and <a href="/facts/Optical_ring_resonators/Jy4NwM7m">optical ring resonators</a>. Through the addition of <a href="/facts/Rare-earth_element/PQSUQumk">rare-earth</a> dopants in the deposition process, Ta2O5 waveguide lasers have been presented for a variety of applications, such as remote sensing and <a href="/facts/Lidar/b9SPUmLM">LiDAR</a>.<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a><a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a><a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> </p> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Fairbrother, Frederick (1967). The Chemistry of Niobium and Tantalum. New York: Elsevier Publishing Company. pp. 1–28. ISBN 978-0-444-40205-9. <a href="978-0-444-40205-9" target="_blank">978-0-444-40205-9</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>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. <a href="9780080529028" target="_blank">9780080529028</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Tantite: Tantite mineral information and data". Mindat.org. Retrieved 2016-03-03. <a href="http://www.mindat.org/min-3884.html" target="_blank">http://www.mindat.org/min-3884.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>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. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Askeljung, Charlotta; Marinder, Bengt-Olov; Sundberg, Margareta (1 November 2003). 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