The scattering of X-radiation by liquid helium II

The unusual physical properties of liquid He ii have led to considerable speculation concerning its internal structure. Since the entropy difference between the liquid and solid states tends to zero as the temperature approaches absolute zero, it appears that in the neighbourhood of zero temperature the liquid must have some kind of ordered structure, and London's theoretical investigations(1) indicated that the best agreement could be obtained with the observed energy and molecular volume by supposing the liquid to have a diamond lattice. On the basis of this work Fröhlich(2) then suggested that in the ordered state He ii possessed a diamond lattice, and that the disordering process consisted in an interchange of the atoms between this lattice and the vacant points of the body-centred cubic lattice from which the diamond lattice may be considered as derived.