3He4He

3He4He refers to a binary liquid mixture of the helium-3 and helium-4 isotopes. It is not a chemical compound; helium isotopes do not form bonds with each other, so a 3He4He mixture is a physical solution rather than a molecule. The isotopes are fully miscible in liquid helium over a wide range of temperatures and pressures, but at sufficiently low temperatures the mixture exhibits a phase separation into two liquid phases: a helium-3–rich (concentrated) phase and a helium-4–rich (dilute) phase.

The phase behavior of the mixture is characterized by a miscibility gap that appears below roughly 0.8–0.9 kelvin, with the exact boundary depending on pressure. Within the two-phase region, the compositions of the two coexisting phases adjust with temperature and pressure, while the system maintains a common temperature. The phenomenon arises from quantum and entropic effects that make the mixed state less favorable at very low temperatures.

A central application of 3He4He is in dilution refrigeration, a method used to reach temperatures of a few millikelvin. In a dilution refrigerator, helium-3 moves between the concentrated and dilute phases. The transfer of He-3 from the concentrated to the dilute phase involves the enthalpy of mixing, which absorbs heat and provides cooling. Continuous circulation of helium-3 between phases enables sustained ultra-low temperatures in experimental devices such as quantum sensors and condensed-mmatter research apparatus.

Production and use involve separating helium-3 from the more abundant helium-4, typically by cryogenic distillation, and then combining controlled amounts to form the desired 3He4He mixture for cryogenic systems. Helium-3 is rarer and more expensive than helium-4, influencing practical applications.