18 Journal of iJuclear Materials 149 (1987) 18-20 North-~oliand. Amsterdam THE DIELECTRIC CONSTANT OF U02 BELOW THE N&EL POINT N. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA HAMPTON and G.A. SAUNDERS School zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Physics, lJniuersity of Bath, Claverton Down, Bath. BAZ 7A Y, United Kingdom J.H. HARDING and A.M. STONEHAM Theoretical Ph_vsics Diuision, A ERE Harwell. Didcot, Uxon, OXI I ORA, kited Kingdom Received 16 December 1986; accepted 7 January 1987 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. 1. The phase transition in UO, Uranium dioxide UO, undergoes a phase transition at about 30 K first detected by a X-type feature in the specific heat [l]. Magnetic susceptibility [2-41 and neu- tron diffraction studies [5] have indicated that the tran- sition is from a high temperature paramagnetic to a low temperature antiferromagnetic state. In the para- magnetic state the crystal structure is fee (fluorite). Below the transition there is a small shift ( - 0.014 A) of the oxygen atoms from the fluorite structure positions [6], the magnetic structure comprising ferromagnetic (001) planes stacked in an alternating plus-minus se- quence (51. The magnetisation changes abruptly at the Neel temperature [5,7]: the transition is first order. At the transition there is a rather abrupt change in length of about 20 ppm and the length continues to decrease rapidly as the temperature is further reduced [8]. The volume change at the transition is consistent with its first order character. Little is known about the electrical properties of the low temperature phase. The present objective has been to measure the dielectric constant of UO, (below 77 K) in both phases to ascertain the effect of the phase transition on this property. A theoretical estimate has also been made to establish whether the structural change could produce a significant effect on the dielectric constant. 2. Measurements of dielectric constant The dielectric constant has been obtained from im- pedance measurements made by an AC bridge tech- nique on a single crystal plate of UO, with evaporated aluminium electrodes. Recent work [9] has identified a boundary layer existing in conjunction with the bulk material for such plate geometry samples. The boundary layer has been found, using impedance plane tech- niques, to dominate the dielectric properties at mod- erate temperatures (300 K) but its effects become in- creasingly less important at lower temperatures. Below 77 K the complex plane profiles show no further evi- dence of any influence due to the boundary layer. Thus at lower temperatures the standard log 2’ versus log 2” profile is a straight line with a gradient of il_,this is expected for the equivalent circuit of a sample whose dielectric properties depend solely on the bulk material [lo]. Furthermore the sample impedance is independent of the applied test voltage over the measurement frequency range 1 kHz to 5MHz; the boundary layer effects should be voltage dependent (111 and have been found to be so in UO, at temperatures in excess of about 150 K ]9]. Thus in the temperature range (below 77 K) of this study, the measured dielectric properties are determined solely by the bulk crystal and the equiv- alent circuit can be represented by a parallel combina-