Editing Helium (section)

===Solid phases===
Helium remains liquid down to [[absolute zero]] at atmospheric pressure, but it freezes at high pressure. Solid helium requires a temperature of 1–1.5&nbsp;K (about −272&nbsp;°C or −457&nbsp;°F) at about 25 bar (2.5&nbsp;MPa) of pressure.<ref>{{cite web|date = 2005-10-05 |url = http://www.phys.ualberta.ca/~therman/lowtemp/projects1.htm |title = Solid Helium |publisher = Department of Physics [[University of Alberta]]|access-date=2008-07-20| archive-url = https://web.archive.org/web/20080531145546/http://www.phys.ualberta.ca/~therman/lowtemp/projects1.htm| archive-date = May 31, 2008}}</ref> It is often hard to distinguish solid from liquid helium since the [[refractive index]] of the two phases are nearly the same. The solid has a sharp [[melting point]] and has a [[crystal]]line structure, but it is highly [[Compressibility|compressible]]; applying pressure in a laboratory can decrease its volume by more than 30%.<ref name="LANL.gov">{{RubberBible86th}}</ref> With a [[bulk modulus]] of about 27 [[megapascal|MPa]]<ref>{{Cite journal|author = Grilly, E. R.|title = Pressure-volume-temperature relations in liquid and solid 4He |journal = Journal of Low Temperature Physics|volume = 11 |issue = 1–2 |pages = 33–52 |doi = 10.1007/BF00655035|date = 1973|bibcode = 1973JLTP...11...33G|s2cid = 189850188 }}</ref> it is ~100 times more compressible than water. Solid helium has a density of {{val|0.214|0.006|u=g/cm<sup>3</sup>}} at 1.15&nbsp;K and 66&nbsp;atm; the projected density at 0&nbsp;K and 25 bar (2.5 MPa) is {{val|0.187|0.009|u=g/cm<sup>3</sup>}}.<ref>{{Cite journal|author = Henshaw, D. B. |title = Structure of Solid Helium by Neutron Diffraction |journal = Physical Review Letters |volume = 109 |issue = 2 |pages = 328–330 |doi = 10.1103/PhysRev.109.328 |date = 1958|bibcode = 1958PhRv..109..328H }}</ref> At higher temperatures, helium will solidify with sufficient pressure. At room temperature, this requires about 114,000 atm.<ref name="Pinceaux1979">{{cite journal |last1=Pinceaux |first1=J.-P. |last2=Maury |first2=J.-P. |last3=Besson |first3=J.-M. |title=Solidification of helium, at room temperature under high pressure |journal=Journal de Physique Lettres |date=1979 |volume=40 |issue=13 |pages=307–308 |doi=10.1051/jphyslet:019790040013030700|s2cid=40164915 |url=https://hal.archives-ouvertes.fr/jpa-00231630/file/ajp-jphyslet_1979_40_13_307_0.pdf }}</ref>

Helium-4 and helium-3 both form several crystalline solid phases, all requiring at least 25&nbsp;bar. They both form an α phase, which has a [[Hexagonal crystal family#Hexagonal close packed|hexagonal close-packed]] (hcp) crystal structure, a β phase, which is [[Cubic crystal system|face-centered cubic]] (fcc), and a γ phase, which is [[Cubic crystal system|body-centered cubic]] (bcc).<ref name="Keller 1969">{{cite book | last=Keller | first=William E. | chapter=Compressed He3 and He4 | title=Helium-3 and Helium-4 | publisher=Springer US | publication-place=Boston, MA | year=1969 | isbn=978-1-4899-6232-4 | doi=10.1007/978-1-4899-6485-4_9 | pages=347–404}}</ref>