THE DIELECTRIC CONSTANT OF U02 BELOW THE N&EL POINT N. HAMPTON and G.A. SAUNDERS

We report measurements of the frequency-dependent dielectric constant of UO, from 4.2 K to above the phase transition at 30 K. The static dielectric constant of 23.6 at 4.2 K is comparable with accepted values at higher temperatures: it is essentially identical in both phases. The effects of undergoing the transition on the dielectric constant are marginal (about 1 I%) and take place in the temperature range 29 K to 37 K. The displacement of the oxygen sublattice, which occurs at the Ntel point, should produce only a 0.05% change on the dielectric constant and of the opposite sense to that measured. Hence the structural changes at the transition are not the primary source of the observed small difference between the dielectric constant in the two phases which probably accrues from the influence of the displacements on a defect-related contribution.