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Solid hydrogen
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<h2 id="molecular-solid-hydrogen">Molecular solid hydrogen</h2> <p>At low temperatures and at pressures up to around 400 GPa (3,900,000 atm), hydrogen forms a series of solid phases formed from discrete H2 molecules. <i>Phase I</i> occurs at low temperatures and pressures, and consists of a hexagonal close-packed array of freely rotating H2 molecules. Upon increasing the pressure at low temperature, a transition to <i>Phase II</i> occurs at up to 110 GPa.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> Phase II is a broken-symmetry structure in which the H2 molecules are no longer able to rotate freely.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> If the pressure is further increased at low temperature, a <i>Phase III</i> is encountered at about 160 GPa. Upon increasing the temperature, a transition to a <i>Phase IV</i> occurs at a temperature of a few hundred kelvin at a range of pressures above 220 GPa.<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> </p><p>Identifying the atomic structures of the different phases of molecular solid hydrogen is extremely challenging, because hydrogen atoms interact with X-rays very weakly and only small samples of solid hydrogen can be achieved in <a href="/facts/Diamond_anvil_cell/vpsaLRrr">diamond anvil cells</a>, so that <a href="/facts/X-ray_diffraction/WtI0F5DN">X-ray diffraction</a> provides very limited information about the structures. Nevertheless, <a href="/facts/Phase_transition/eLP2BY8R">phase transitions</a> can be detected by looking for abrupt changes in the <a href="/facts/Raman_spectroscopy/XY063k29">Raman spectra</a> of samples. Furthermore, atomic structures can be inferred from a combination of experimental Raman spectra and first-principles modelling.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> <a href="/facts/Density_functional_theory/wwz4Gqdg">Density functional theory</a> calculations have been used to search for candidate atomic structures for each phase. These candidate structures have low free energies and Raman spectra in agreement with the experimental spectra.<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><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> <a href="/facts/Quantum_Monte_Carlo/qenhfw9u">Quantum Monte Carlo</a> methods together with a first-principles treatment of anharmonic vibrational effects have then been used to obtain the relative <a href="/facts/Gibbs_free_energies/bvenDuzh">Gibbs free energies</a> of these structures and hence to obtain a theoretical pressure-temperature phase diagram that is in reasonable quantitative agreement with experiment.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> On this basis, Phase II is believed to be a molecular structure of <i>P</i>21/<i>c</i> symmetry; Phase III is (or is similar to) a structure of <i>C</i>2/<i>c</i> symmetry consisting of flat layers of molecules in a distorted hexagonal arrangement; and Phase IV is (or is similar to) a structure of <i>Pc</i> symmetry, consisting of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Compressed_hydrogen/SgIHnnFc">Compressed hydrogen</a></li> <li><a href="/facts/Liquid_hydrogen/Vpk3Gb8P">Liquid hydrogen</a></li> <li><a href="/facts/Metallic_hydrogen/Wc4P3R6I">Metallic hydrogen</a></li> <li><a href="/facts/Slush_hydrogen/BMKN4MwV">Slush hydrogen</a></li> <li><a href="/facts/Timeline_of_hydrogen_technologies/fbaM5Q6y">Timeline of hydrogen technologies</a></li></ul> <h2 id="further-reading">Further reading</h2> <ul><li><a href="https://web.archive.org/web/20110520115418/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0748847">Melting Characteristics and Bulk Thermophysical Properties of Solid Hydrogen</a>", <a href="/facts/Air_Force_Rocket_Propulsion_Laboratory/MwAW8n6a">Air Force Rocket Propulsion Laboratory</a>, Technical Report, 1972</li></ul> <h2 id="external-links">External links</h2> <ul><li>"<a href="https://web.archive.org/web/20090312052336/http://sciencelinks.jp/j-east/article/200211/000020021102A0308056.php">Properties of solid hydrogen at very low temperatures</a>" (2001)</li> <li>"<a href="https://web.archive.org/web/20110927045420/http://gltrs.grc.nasa.gov/reports/2002/TM-2002-211915.pdf">Solid hydrogen experiments for atomic propellants</a>"</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Correspondence and General A-I DEWAR/Box D I <a href="http://www.nationalarchives.gov.uk/A2A/records.aspx?cat=116-dewar_1&cid=-1&Gsm=2008-06-18#-1" target="_blank">http://www.nationalarchives.gov.uk/A2A/records.aspx?cat=116-dewar_1&cid=-1&Gsm=2008-06-18#-1</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Dewar, James (1899). "Sur la solidification de l'hydrogène". Annales de Chimie et de Physique. 18: 145–150. <a href="http://gallica.bnf.fr/ark:/12148/bpt6k349183/f143.table" target="_blank">http://gallica.bnf.fr/ark:/12148/bpt6k349183/f143.table</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Mao, Ho-kwang; Hemley, Russell J. (1 April 1994). "Ultrahigh-pressure transitions in solid hydrogen". Reviews of Modern Physics. 66 (2): 671–692. Bibcode:1994RvMP...66..671M. doi:10.1103/RevModPhys.66.671. <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>Goncharenko, Igor; Loubeyre, Paul (30 June 2005). "Neutron and X-ray diffraction study of the broken symmetry phase transition in solid deuterium". Nature. 435 (7046): 1206–1209. Bibcode:2005Natur.435.1206G. doi:10.1038/nature03699. PMID 15988519. S2CID 4416401. <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>Howie, Ross T.; Guillaume, Christophe L.; Scheler, Thomas; Goncharov, Alexander F.; Gregoryanz, Eugene (19 March 2012). "Mixed Molecular and Atomic Phase of Dense Hydrogen". Physical Review Letters. 108 (12): 125501. Bibcode:2012PhRvL.108l5501H. doi:10.1103/PhysRevLett.108.125501. PMID 22540596. <a href="https://doi.org/10.1103%2FPhysRevLett.108.125501" target="_blank">https://doi.org/10.1103%2FPhysRevLett.108.125501</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Eremets, Mikhail I.; Troyan, I. A. (13 November 2011). "Conductive dense hydrogen". Nature Materials. 10 (12): 927–931. Bibcode:2011NatMa..10..927E. doi:10.1038/nmat3175. PMID 22081083. <a href="/wiki/Mikhail_Eremets" target="_blank">/wiki/Mikhail_Eremets</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>McMahon, Jeffrey M.; Morales, Miguel A.; Pierleoni, Carlo; Ceperley, David M. (13 November 2012). "The properties of hydrogen and helium under extreme conditions" (PDF). Reviews of Modern Physics. 84 (4): 1607–1653. Bibcode:2012RvMP...84.1607M. doi:10.1103/RevModPhys.84.1607. OSTI 1107313. <a href="https://digital.library.unt.edu/ark:/67531/metadc864848/m2/1/high_res_d/1107313.pdf" target="_blank">https://digital.library.unt.edu/ark:/67531/metadc864848/m2/1/high_res_d/1107313.pdf</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Pickard, Chris J.; Needs, Richard J. (27 May 2007). "Structure of phase III of solid hydrogen". Nature Physics. 3 (7): 473–476. Bibcode:2007NatPh...3..473P. doi:10.1038/nphys625. <a href="https://doi.org/10.1038%2Fnphys625" target="_blank">https://doi.org/10.1038%2Fnphys625</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Pickard, Chris J.; Needs, Richard J. (17 February 2009). "Structures at high pressure from random searching". Physica Status Solidi B. 246 (3): 536–540. Bibcode:2009PSSBR.246..536P. doi:10.1002/pssb.200880546. S2CID 97258049. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Pickard, Chris J.; Martinez-Canales, Miguel; Needs, Richard J. (15 June 2012). "Density functional theory study of phase IV of solid hydrogen". Physical Review B. 85 (21): 214114. arXiv:1204.3304. Bibcode:2012PhRvB..85u4114P. doi:10.1103/PhysRevB.85.214114. S2CID 119269630. <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>Drummond, N. D.; Monserrat, B.; Lloyd-Williams, J. H.; Lopez Rios, P.; Pickard, Chris J.; Needs, Richard J. (28 July 2015). "Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures". Nature Communications. 6: 7794. arXiv:1508.02313. Bibcode:2015NatCo...6.7794D. doi:10.1038/ncomms8794. PMC 4525154. PMID 26215251. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4525154" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4525154</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> </ol>