Bond-Lengths in Solid Helium

RECENTLY, Vignos and Fairbank 1 reported the existence of a new solid phase of helium-4 which exists in a narrow range of pressure between 1.45° K. and 1.78° K. They have designated the new solid as the γ-phase. Its density is less than that of the α-phase (hexagonal closest packing) of helium with which it is contiguous. It is known 2,3 that in helium-3 at a temperature almost as low, there is a phase transition in the solid from hexagonal closest packing to a less-dense phase which is body-centred cubic. Reasoning by analogy, the authors believe the new phase will be found to have the body-centred cubic, A 2, structure.