Iron-based superconductor

<p>Iron-based superconductors (FeSC) are <a href="/facts/Iron/QK1JYy8E">iron</a>-containing chemical compounds whose <a href="/facts/Superconductors/pqQAlSzJ">superconducting</a> properties were discovered in 2006. The first of such superconducting compounds belong to the group of <a href="/facts/Oxypnictide/oJGSPFw0">oxypnictides</a>, which was known since 1995. Until 2006, however, they were in the first stages of experimentation and implementation and only the semiconductive properties of these compounds were known and patented. Previously most <a href="/facts/High-temperature_superconductor/MXtx2gvV">high-temperature superconductors</a> were <a href="/facts/Cuprate_superconductor/U0MqqhnQ">cuprates</a> containing <a href="/facts/Copper/I8PEE8oX">copper</a> - <a href="/facts/Oxygen/acyn6o8r">oxygen</a> layers. Much of the interest in iron-based superconductors is precisely because of the differences from the cuprates, which may help lead to a theory of non-<a href="/facts/BCS-theory/KmlyhxPS">BCS-theory</a> superconductivity.
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Iron-based superconductors of the group of <a href="/facts/Oxypnictide/oJGSPFw0">oxypnictides</a> were initially called ferropnictides. The crystal structure of these compounds displays conducting layers of <a href="/facts/Iron/QK1JYy8E">iron</a> and a <a href="/facts/Pnictide/jX4hvCkK">pnictogen</a> (typically <a href="/facts/Arsenic/k1c4f44D">arsenic</a> (As) and <a href="/facts/Phosphorus/27xCeIqs">phosphorus</a> (P)) separated by a charge-reservoir block. It has also been found that some iron <a href="/facts/Chalcogen/nXLGmnqL">chalcogens</a> and <a href="/facts/Crystallogen/PkWCV3Vx">crystallogens</a> superconduct. </p><p>The crystalline material, known chemically as LaOFeAs, stacks iron and arsenic layers, where the electrons flow, between planes of <a href="/facts/Lanthanum/Uhmmy1sB">lanthanum</a> and <a href="/facts/Oxygen/acyn6o8r">oxygen</a>. Replacing up to 11 percent of the oxygen with <a href="/facts/Fluorine/YUV0Fb8m">fluorine</a> improved the compound – it became superconductive at 26 <a href="/facts/Kelvin/E3trzC4C">kelvin</a>, the team reports in the March 19, 2008 Journal of the American Chemical Society. Subsequent research from other groups suggests that replacing the lanthanum in LaOFeAs with other rare earth elements such as <a href="/facts/Cerium/a7UjMBQ6">cerium</a>, <a href="/facts/Samarium/09K3kl93">samarium</a>, <a href="/facts/Neodymium/GzDkTIoI">neodymium</a> and <a href="/facts/Praseodymium/2lnRJ9dH">praseodymium</a> leads to superconductors that work at 52 kelvin.</p>
<p>Iron-based superconductors are classified according to their crystal structure and chemical formula into the following main families,
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<ul><li>1111-type, with representative compounds LaFePO, LaFeAsO, SmFeAsO, PrFeAsO, and LaFeSiH.</li>
<li>111-type such as LiFeAs, NaFeAs, and LiFeP.</li>
<li>11-type FeSe</li>
<li>122-type such as BaFe2As2, SrFe2As2 and CaFe2As2</li></ul>
<p>Superconductivity is obtained either in the parent phases of some of these systems (e.g. LaFePO, LaFeSiH, and LiFeAs) or by means of doping or applied pressure. 
</p><p>Undoped <i>β</i>-FeSe is the simplest iron-based superconductor but with distinct properties. It has a <a href="/facts/Superconductivity/pqQAlSzJ">critical temperature (<i>T</i>c)</a> of 8 <a href="/facts/Kelvin/E3trzC4C">K</a> at normal pressure, and 36.7 K under high pressure and by means of intercalation. The combination of both intercalation and higher pressure results in re-emerging superconductivity at <i>T</i>c of up to 48 K (see, and references therein).
</p><p>Compared with other families, the synthesis of the <a href="/facts/122_iron_arsenide/s9dpGfk3">122 compounds</a> is relatively easy which facilitates the investigation of these systems.
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<table><tbody><tr><th><a href="/facts/Oxypnictide/oJGSPFw0">Oxypnictide</a></th><th><i>T</i>c (K)</th></tr><tr><td>LaO0.89F0.11FeAs</td><td>26</td></tr><tr><td>LaO0.9F0.2FeAs</td><td>28.5</td></tr><tr><td>CeFeAsO0.84F0.16</td><td>41</td></tr><tr><td>SmFeAsO0.9F0.1</td><td>43</td></tr><tr><td>La0.5Y0.5FeAsO0.6</td><td>43.1</td></tr><tr><td>NdFeAsO0.89F0.11</td><td>52</td></tr><tr><td>PrFeAsO0.89F0.11</td><td>52</td></tr><tr><td>ErFeAsO1−y</td><td>45</td></tr><tr><td>Al-32522 (CaAlOFeAs)</td><td>30(As), 16.6 (P)</td></tr><tr><td>Al-42622 (CaAlOFeAs)</td><td>28.3(As), 17.2 (P)</td></tr><tr><td>GdFeAsO0.85</td><td>53.5</td></tr><tr><td>BaFe1.8Co0.2As2</td><td>25.3</td></tr><tr><td>SmFeAsO~0.85</td><td></td></tr></tbody></table>
<table><tbody><tr><th>Non-oxypnictide</th><th><i>T</i>c (K)</th></tr><tr><td>Ba0.6K0.4Fe2As2</td><td>38</td></tr><tr><td>Ca0.6Na0.4Fe2As2</td><td>26</td></tr><tr><td>CaFe0.9Co0.1AsF</td><td>22</td></tr><tr><td>Sr0.5Sm0.5FeAsF</td><td>56</td></tr><tr><td>LiFeAs</td><td>18</td></tr><tr><td>NaFeAs</td><td>9–25</td></tr><tr><td><a href="/facts/Iron(II)_selenide/p4JLaLJo">FeSe</a></td><td><27</td></tr><tr><td>LaFeSiH</td><td>11</td></tr></tbody></table>

<p>Compounds such as Sr2ScFePO3 discovered in 2009 are referred to as the '42622' family, as FePSr2ScO3. Noteworthy is the synthesis of (Ca4Al2O6−y)(Fe2Pn2) (or Al-42622(Pn); Pn = As and P) using high-pressure synthesis technique. Al-42622(Pn) exhibit superconductivity for both Pn = As and P with the transition temperatures of 28.3 K and 17.1 K, respectively. The a-lattice parameters of Al-42622(Pn) (a = 3.713 Å and 3.692 Å for Pn = As and P, respectively) are smallest among the iron-pnictide superconductors. Correspondingly, Al-42622(As) has the smallest As–Fe–As bond angle (102.1°) and the largest As distance from the Fe planes (1.5 Å). High-pressure technique also yields (Ca3Al2O5−y)(Fe2Pn2) (Pn = As and P), the first reported iron-based superconductors with the perovskite-based '32522' structure. The transition temperature (Tc) is 30.2 K for Pn = As and 16.6 K for Pn = P. The emergence of superconductivity is ascribed to the small tetragonal a-axis lattice constant of these materials. From these results, an empirical relationship was established between the a-axis lattice constant and Tc in iron-based superconductors.
</p><p>In 2009, it was shown that undoped iron pnictides had a magnetic quantum critical point deriving from competition between electronic localization and itinerancy.
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Iron-based superconductor open-in-new

Iron-based superconductor