Helium refuses to freeze
Helium-4 at ordinary pressure stays liquid down to absolute zero. It does not become a solid unless the pressure exceeds about 25 atmospheres.
Established2 min read
Kamerlingh Onnes liquefied helium in 1908. Cooling further did not freeze it. The atoms are light and the forces between them are weak, so the zero-point energy — the motion quantum mechanics will not let you remove — is enough to keep the liquid from locking into a lattice.
Squeeze it above about 25 bar and the lattice wins. There is a solid. At one bar there is not, not at any temperature. The phase diagram has a liquid that touches zero.
Caveats
- This is helium at saturated vapour pressure or one atmosphere. Under pressure it does freeze.
- Helium-3, the lighter isotope, also refuses, and its diagram is stranger.
Sources
- 1Donnelly R. J., Barenghi C. F.. The Observed Properties of Liquid Helium at the Saturated Vapor Pressure. Journal of Physical and Chemical Reference Data 27, 1998.