Abstract
The 'crystal structure problem' in the heavier rare-gas solids is briefly reviewed. New calculations are presented which indicate that the relative free-energy difference between the FCC and HCP phases, Delta , is approximately=(2-5)*10-3 for Xe at low temperatures, falling with increasing temperature. New experimental measurements by transmission electron microscopy of the stacking fault energy gamma are reported. The parameters gamma and Delta are related by a simple model, and Delta is thereby found to be approximately=(6+or-2)*10-3 for Xe at T=25K, falling to (4+or-2)*10-3 at 55K. A measurement for Kr is reported as Delta =(4.6+or-1.2)*10-3 at 40K. The measurements for Xe constitute the first tentative evidence that gamma and Delta decrease with increasing T in the sense indicated by calculation.