X-ray resonant scattering determination of the antiferroquadrupolar ordering in UPd3 at low temperatures

We have used x-ray resonant scattering techniques to make a comprehensive study of the quadrupolar order in UPd3. New measurements reveal that the order parameter describing the in-phase stacking along the c-axis of the quadrupole moments of the uranium ions on the quasi-cubic sites is primarily Q(xy) in the two lowest temperature phases. These results are discussed within the context of a three-level crystal field model for the uranium 5f(2) electrons, the allowed quadrupolar operators, and the constraints deduced from the analysis of heat capacity measurements. We have now deduced, and summarize, the order parameters for each of the four antiferroquadrupolar phases of UPd3.