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Graphite intercalation compound
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<h2 id="preparation-and-structure">Preparation and structure</h2> <p>These materials are prepared by treating graphite with a strong oxidant or a strong reducing agent: </p> C + <i>m</i> X → CX<i>m</i> <p>The reaction is reversible. </p><p>The host (graphite) and the guest X interact by <a href="/facts/Charge_transfer_complex/hbIyNJlo">charge transfer</a>. An analogous process is the basis of commercial <a href="/facts/Lithium-ion_battery/3L22xFhB">lithium-ion batteries</a>. </p><p>In a graphite intercalation compound not every layer is necessarily occupied by guests. In so-called <i>stage 1 compounds</i>, graphite layers and intercalated layers alternate and in <i>stage 2 compounds</i>, two graphite layers with no guest material in between alternate with an intercalated layer. The actual composition may vary and therefore these compounds are an example of <a href="/facts/Stoichiometry/I5p9cJzV">non-stoichiometric</a> compounds. It is customary to specify the composition together with the stage. The layers are pushed apart upon incorporation of the guest ions. </p> <h2 id="examples">Examples</h2> <h3>Alkali and alkaline earth derivatives</h3> <p>One of the best studied graphite intercalation compounds, KC8, is prepared by melting <a href="/facts/Potassium/FWPwS7Rj">potassium</a> over graphite powder. The potassium is absorbed into the graphite and the material changes color from black to bronze.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> The resulting solid is <a href="/facts/Pyrophoric/1b8Sl2AJ">pyrophoric</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> The composition is explained by assuming that the potassium to potassium distance is twice the distance between hexagons in the carbon framework. The bond between anionic graphite layers and potassium cations is ionic. The electrical conductivity of the material is greater than that of α-graphite.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> KC8 is a <a href="/facts/Superconductor/pqQAlSzJ">superconductor</a> with a very low critical temperature Tc = 0.14 K.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> Heating KC8 leads to the formation of a series of decomposition products as the K atoms are eliminated: </p> 3 KC8 → KC24 + 2 K <p>Via the intermediates KC24 (blue in color),<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> KC36, KC48, ultimately the compound KC60 results. </p><p>The stoichiometry MC8 is observed for M = K, Rb and Cs. For smaller ions M = Li+, Sr2+, Ba2+, Eu2+, Yb3+, and Ca2+, the limiting stoichiometry is MC6.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> Calcium graphite CaC6 is obtained by immersing highly oriented <a href="/facts/Pyrolytic_graphite/qwrbjCct">pyrolytic graphite</a> in liquid Li–Ca alloy for 10 days at 350 °C. The crystal structure of CaC6 belongs to the R3m space group. The graphite interlayer distance increases upon Ca intercalation from 3.35 to 4.524 Å, and the carbon-carbon distance increases from 1.42 to 1.444 Å. </p> <p>With <a href="/facts/Barium/IkL09XHQ">barium</a> and <a href="/facts/Ammonia/MtLtJFtz">ammonia</a>, the cations are solvated, giving the stoichiometry (Ba(NH3)2.5C10.9(stage 1)) or those with <a href="/facts/Caesium/dXKN1S5k">caesium</a>, <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> and <a href="/facts/Potassium/FWPwS7Rj">potassium</a> (CsC8·K2H4/3C8(stage 1)). </p><p>In situ adsorption on free-standing graphene and intercalation in bilayer graphene of the alkali metals K, Cs, and Li was observed by means of low-energy electron microscopy.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p><p>Different from other alkali metals, the amount of Na intercalation is very small. Quantum-mechanical calculations show that this originates from a quite general phenomenon: among the alkali and alkaline earth metals, Na and Mg generally have the weakest chemical binding to a given substrate, compared with the other elements in the same group of the periodic table.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> The phenomenon arises from the competition between trends in the ionization energy and the ion–substrate coupling, down the columns of the periodic table.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> However, considerable Na intercalation into graphite can occur in cases when the ion is wrapped in a solvent shell through the process of co-intercalation. A complex magnesium(I) species has also been intercalated into graphite.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p> <h3>Graphite bisulfate, perchlorate, hexafluoroarsenate: oxidized carbons</h3> <p>The intercalation compounds graphite bisulfate and graphite perchlorate can be prepared by treating graphite with strong oxidizing agents in the presence of strong acids. In contrast to the potassium and calcium graphites, the carbon layers are oxidized in this process: </p> 48 C + 0. 5 ［O ］+ 3 H2SO4 → [C24]+[HSO4]−·2H2SO4 + 0.5 H2O <p>In graphite perchlorate, planar layers of carbon atoms are 794 <a href="/facts/Picometre/Cb54mVBX">picometers</a> apart, separated by ClO−4 ions. Cathodic reduction of graphite perchlorate is analogous to heating KC8, which leads to a sequential elimination of HClO4. </p><p>Both graphite bisulfate and graphite perchlorate are better conductors as compared to graphite, as predicted by using a positive-hole mechanism.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> Reaction of graphite with [O2]+[AsF6]− affords the salt [C8]+[AsF6]−.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p> <h3>Metal halide derivatives</h3> <p>A number of metal halides intercalate into graphite. The chloride derivatives have been most extensively studied. Examples include MCl2 (M = Zn, Ni, Cu, Mn), MCl3 (M = Al, Fe, Ga), MCl4 (M = Zr, Pt), etc.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> The materials consists of layers of close-packed metal halide layers between sheets of carbon. The derivative C~8FeCl3 exhibits <a href="/facts/Spin_glass/v2Cxng9Y">spin glass</a> behavior.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> It proved to be a particularly fertile system on which to study phase transitions. A stage n magnetic graphite intercalation compounds has n graphite layers separating successive magnetic layers. As the stage number increases the interaction between spins in successive magnetic layers becomes weaker and 2D magnetic behaviour may arise. </p> <h3>Halogen- and oxide-graphite compounds</h3> <p>Chlorine and bromine reversibly intercalate into graphite. Iodine does not. Fluorine reacts irreversibly. In the case of bromine, the following stoichiometries are known: C<i>n</i>Br for <i>n</i> = 8, 12, 14, 16, 20, and 28. </p><p>Because it forms irreversibly, <a href="/facts/Carbon_monofluoride/m2J1ApcP">carbon monofluoride</a> is often not classified as an intercalation compound. It has the formula (CF)<i>x</i>. It is prepared by reaction of gaseous <a href="/facts/Fluorine/YUV0Fb8m">fluorine</a> with graphitic carbon at 215–230 °C. The color is greyish, white, or yellow. The bond between the carbon and fluorine atoms is covalent. Tetracarbon monofluoride (C4F) is prepared by treating graphite with a mixture of fluorine and <a href="/facts/Hydrogen_fluoride/XfQxlqR6">hydrogen fluoride</a> at room temperature. The compound has a blackish-blue color. Carbon monofluoride is not electrically conductive. It has been studied as a <a href="/facts/Cathode/tqdlGayU">cathode</a> material in one type of primary (non-rechargeable) <a href="/facts/Lithium_battery/D5yfo8XS">lithium batteries</a>. </p><p><a href="/facts/Graphite_oxide/mxo0EJJl">Graphite oxide</a> is an unstable yellow solid. </p> <h2 id="properties-and-applications">Properties and applications</h2> <p>Graphite intercalation compounds have fascinated materials scientists for many years owing to their diverse electronic and electrical properties. </p> <h3>Superconductivity</h3> <p>Among the superconducting graphite intercalation compounds, CaC6 exhibits the highest <a href="/facts/Superconductivity/pqQAlSzJ">critical temperature</a> <i>T</i>c = 11.5 K, which further increases under applied pressure (15.1 K at 8 GPa).<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> Superconductivity in these compounds is thought to be related to the role of an interlayer state, a free electron like band lying roughly 2 eV (0.32 aJ) above the <a href="/facts/Fermi_level/WpyRA0V7">Fermi level</a>; superconductivity only occurs if the interlayer state is occupied.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> Analysis of pure CaC6 using a high quality <a href="/facts/Ultraviolet_light/kncBUqn5">ultraviolet light</a> revealed to conduct <a href="/facts/Angle-resolved_photoemission_spectroscopy/vQW4b9Dh">angle-resolved photoemission spectroscopy</a> measurements. The opening of a superconducting gap in the π* band revealed a substantial contribution to the total electron–phonon-coupling strength from the π*-interlayer interband interaction.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> </p> <h3>Reagents in chemical synthesis: KC8</h3> <p>The bronze-colored material KC8 is one of the strongest <a href="/facts/Reducing_agents/CtfSGVXC">reducing agents</a> known. It has also been used as a <a href="/facts/Catalyst/LPBS7TQC">catalyst</a> in <a href="/facts/Polymerization/1hmBz7Ur">polymerizations</a> and as a <a href="/facts/Coupling_reaction/L464wvvk">coupling reagent</a> for <a href="/facts/Aryl_halide/zEQlrpun">aryl halides</a> to <a href="/facts/Biphenyl/WWuTjvpd">biphenyls</a>.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> In one study, freshly prepared KC8 was treated with 1-iodododecane delivering a modification (<a href="/facts/Micrometre/7eDFxD2L">micrometre</a> scale carbon platelets with long alkyl chains sticking out providing solubility) that is soluble in <a href="/facts/Chloroform/gvzuJMXa">chloroform</a>.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> Another potassium graphite compound, KC24, has been used as a neutron monochromator. A new essential application for potassium graphite was introduced by the invention of the <a href="/facts/Potassium-ion_battery/aBonvKKe">potassium-ion battery</a>. Like the <a href="/facts/Lithium-ion_battery/3L22xFhB">lithium-ion battery</a>, the <a href="/facts/Potassium-ion_battery/aBonvKKe">potassium-ion battery</a> should use a carbon-based anode instead of a metallic anode. In this circumstance, the stable structure of potassium graphite is an important advantage. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Buckminsterfullerene_intercalates/SFPFuYtS">Buckminsterfullerene intercalates</a></li> <li><a href="/facts/Covalent_superconductors/0ujLMbAd">Covalent superconductors</a></li> <li><a href="/facts/Magnesium_diboride/fsgNsNPP">Magnesium diboride</a>, which uses hexagonal planar <a href="/facts/Boron/Pmwgd7Mf">boron</a> sheets instead of carbon</li> <li><a href="/facts/Pyrolytic_graphite/qwrbjCct">Pyrolytic graphite</a></li></ul> <h2 id="further-reading">Further reading</h2> <ul><li>T. Enoki, M. Suzuki and M. Endo (2003). <i>Graphite intercalation compounds and applications</i>. Oxford University Press. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-19-512827-7.</li> <li><a href="/facts/Mildred_Dresselhaus/g6JjzPGs">Dresselhaus, M.S.</a>; Dresselhaus, G. (1981). "Intercalation compounds of graphite". <i>Advances in Physics</i>. 30 (2): 139–326. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1981AdPhy..30..139D">1981AdPhy..30..139D</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1080%2F00018738100101367">10.1080/00018738100101367</a>. (187 pages), also reprinted as Dresselhaus, M. S.; Dresselhaus, G. (2002). "Intercalation compounds of graphite". <i>Advances in Physics</i>. 51 (1): 1–186. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2002AdPhy..51....1D">2002AdPhy..51....1D</a>. <a href="/facts/CiteSeerX_(identifier)/SceDmd3c">CiteSeerX</a> <a href="https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.170.2655">10.1.1.170.2655</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1080%2F00018730110113644">10.1080/00018730110113644</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:123597602">123597602</a>.</li> <li>D. Savoia; Trombini, C.; Umani-Ronchi, A.; et al. (1985). <a href="http://www.iupac.org/publications/pac/1985/pdf/5712x1887.pdf">"Applications of potassium-graphite and metals dispersed on graphite in organic synthesis"</a> (PDF). <i>Pure and Applied Chemistry</i> (PDF). 57 (12): 1887. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1351%2Fpac198557121887">10.1351/pac198557121887</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:95591721">95591721</a>.</li> <li>Suzuki, Itsuko S.; Ting-Yu Huang; Masatsugu Suzuki (13 June 2002). "Magnetic phase diagram of the stage-1 CoCl2 graphite intercalation compound: Existence of metamagnetic transition and spin-flop transitions". <i>Physical Review B</i>. 65 (22): 224432. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2002PhRvB..65v4432S">2002PhRvB..65v4432S</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1103%2FPhysRevB.65.224432">10.1103/PhysRevB.65.224432</a>.</li> <li>Rancourt, DG; C Meschi; S Flandrois (1986). "S=1/2 antiferromagnetic finite chains effectively isolated by frustration: CuCl2-intercalated graphite". <i>Physical Review B</i>. 33 (1): 347–355. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1986PhRvB..33..347R">1986PhRvB..33..347R</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1103%2FPhysRevB.33.347">10.1103/PhysRevB.33.347</a>. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/9937917">9937917</a>.</li></ul> Wikimedia Commons has media related to Graphite intercalation compounds. <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. <a href="978-0-08-037941-8" target="_blank">978-0-08-037941-8</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>H-P Boehm; Setton, R.; Stumpp, E.; et al. (1994). "Nomenclature and terminology of graphite intercalation compounds" (PDF). Pure and Applied Chemistry (PDF). 66 (9): 1893. doi:10.1351/pac199466091893. S2CID 98227391. Archived from the original (PDF) on 2012-04-06. <a href="https://web.archive.org/web/20120406105828/http://www.iupac.org/publications/pac/1994/pdf/6609x1893.pdf" target="_blank">https://web.archive.org/web/20120406105828/http://www.iupac.org/publications/pac/1994/pdf/6609x1893.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Ottmers, D.M.; Rase, H.F. (1966). "Potassium graphites prepared by mixed-reaction technique". Carbon. 4 (1): 125–127. Bibcode:1966Carbo...4..125O. doi:10.1016/0008-6223(66)90017-0. ISSN 0008-6223. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 14: The group 14 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 386. ISBN 978-0-13-175553-6. <a href="978-0-13-175553-6" target="_blank">978-0-13-175553-6</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 14: The group 14 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 386. ISBN 978-0-13-175553-6. <a href="978-0-13-175553-6" target="_blank">978-0-13-175553-6</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>NIST Ionizing Radiation Division 2001 – Major Technical Highlights. physics.nist.gov <a href="https://web.archive.org/web/20061006232058/http://physics.nist.gov/TechAct.2001/Div846/div846h.html" target="_blank">https://web.archive.org/web/20061006232058/http://physics.nist.gov/TechAct.2001/Div846/div846h.html</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Emery, N.; Hérold, Claire; Marêché, Jean-François; Lagrange, Philippe; et al. (2008). "Review: Synthesis and superconducting properties of CaC6". Science and Technology of Advanced Materials (PDF). 9 (4): 044102. Bibcode:2008STAdM...9d4102E. doi:10.1088/1468-6996/9/4/044102. PMC 5099629. PMID 27878015. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5099629" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5099629</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Ottmers, D.M.; Rase, H.F. (1966). "Potassium graphites prepared by mixed-reaction technique". Carbon. 4 (1): 125–127. Bibcode:1966Carbo...4..125O. doi:10.1016/0008-6223(66)90017-0. ISSN 0008-6223. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Emery, N.; Hérold, Claire; Marêché, Jean-François; Lagrange, Philippe; et al. (2008). "Review: Synthesis and superconducting properties of CaC6". Science and Technology of Advanced Materials (PDF). 9 (4): 044102. Bibcode:2008STAdM...9d4102E. doi:10.1088/1468-6996/9/4/044102. PMC 5099629. PMID 27878015. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5099629" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5099629</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Lorenzo, Marianna; Escher, Conrad; Latychevskaia, Tatiana; Fink, Hans-Werner (2018-05-07). "Metal Adsorption and Nucleation on Free-Standing Graphene by Low-Energy Electron Point Source Microscopy". Nano Letters. 18 (6). American Chemical Society (ACS): 3421–3427. arXiv:2301.10548. Bibcode:2018NanoL..18.3421L. doi:10.1021/acs.nanolett.8b00359. PMID 29733660. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Liu, Yuanyue; Merinov, Boris V.; Goddard, William A. (5 April 2016). "Origin of low sodium capacity in graphite and generally weak substrate binding of Na and Mg among alkali and alkaline earth metals". Proceedings of the National Academy of Sciences. 113 (14): 3735–3739. arXiv:1604.03602. Bibcode:2016PNAS..113.3735L. doi:10.1073/pnas.1602473113. PMC 4833228. PMID 27001855. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4833228" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4833228</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Liu, Yuanyue; Merinov, Boris V.; Goddard, William A. (5 April 2016). "Origin of low sodium capacity in graphite and generally weak substrate binding of Na and Mg among alkali and alkaline earth metals". Proceedings of the National Academy of Sciences. 113 (14): 3735–3739. arXiv:1604.03602. Bibcode:2016PNAS..113.3735L. doi:10.1073/pnas.1602473113. PMC 4833228. PMID 27001855. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4833228" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4833228</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Xu, Wei; Zhang, Hanyang; Lerner, Michael M. (2018-06-25). "Graphite Intercalation by Mg Diamine Complexes". Inorganic Chemistry. 57 (14). American Chemical Society (ACS): 8042–8045. doi:10.1021/acs.inorgchem.8b01250. ISSN 0020-1669. PMID 29939016. S2CID 49412174. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 14: The group 14 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 386. ISBN 978-0-13-175553-6. <a href="978-0-13-175553-6" target="_blank">978-0-13-175553-6</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 14: The group 14 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 386. ISBN 978-0-13-175553-6. <a href="978-0-13-175553-6" target="_blank">978-0-13-175553-6</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. <a href="978-0-08-037941-8" target="_blank">978-0-08-037941-8</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>Millman, S E; Zimmerman, G O (1983). "Observation of spin glass state in FeCl3: intercalated graphite". Journal of Physics C: Solid State Physics. 16 (4): L89. 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