It was pointed out that, in the case of a highly compressible crystal, the direct method for measuring thermal vacancy contents (by the comparison of X-ray and macroscopic cell volumes over a range of temperatures) could be replaced by X-ray measurements alone which were performed on a sample that was constrained to constant volume. For both methods, the results were independent of the assumptions made concerning the nature and strength of the distortions that were introduced into the crystalline lattice by the presence of point defects. This was especially important in quantum-solid He, where some defects could be non-localized. It was noted that X-ray measurements of this type had been reported for many different solid He phases, such as body-centered cubic 3He, hexagonal close-packed 3He, body-centered cubic 4He, hexagonal close-packed 4He, and body-centered cubic 4He-3He mixtures. The various He solids had identical interatomic interactions, but differed with regard to their statistics and other properties and provided a rich family of crystals for study. However, some of the vacancy contents that were measured directly had proved difficult to reconcile with less direct information which was deduced from other types of measurement, such as heat capacity and pressure (which reflected equilibrium properties) and nuclear magnetic resonance relaxation and ionic conduction (which were related to defect diffusion).

R.O.Simmons: Journal of the Physics and Chemistry of Solids, 1994, 55[10], 895-906