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Molecular orientations are revealed to arrange orderly in the [0001] and [112\u00af0] lattice directions in phase II although it has a lower-symmetry P63\/mmc lattice structure than phase I. Consequently, both molecular centers and orientations tend to stack in the ABAB fashion in the [0001] lattice direction when solid hydrogen transits from phase I to II. By monitoring the evolution of infrared frequency with temperature at different pressures, the critical pressure at which solid hydrogen is initiated to transit from phase I to II is determined at different temperatures, especially 55\u00a0GPa at 0\u00a0K. This finding is expected to resolve the long-term confusion on the structure of phase II and provides new insight into solid hydrogen at low temperature, especially involving the arrangement nature of molecules therein.<\/jats:p>","DOI":"10.1063\/5.0337016","type":"journal-article","created":{"date-parts":[[2026,9,1]],"date-time":"2026-09-01T12:21:22Z","timestamp":1788265282000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Structural transition of solid hydrogen between phases I and II at low temperatures"],"prefix":"10.1063","volume":"165","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-3492-4239","authenticated-orcid":false,"given":"Haile","family":"Lei","sequence":"first","affiliation":[{"name":"Laser Fusion Research Center, China Academy of Engineering Physics , 621900 Mianyang,","place":["China"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0009-5699-6162","authenticated-orcid":false,"given":"Junnan","family":"Bai","sequence":"additional","affiliation":[{"name":"Laser Fusion Research Center, China Academy of Engineering Physics , 621900 Mianyang,","place":["China"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"317","published-online":{"date-parts":[[2026,9,1]]},"reference":[{"key":"2026090108201554700_c1","doi-asserted-by":"publisher","first-page":"016901","DOI":"10.1088\/0034-4885\/73\/1\/016901","article-title":"The physical properties of extra-solar planets","volume":"73","year":"2010","journal-title":"Rep. 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