A new model for the crystal field and the quadrupolar phase transitions of UPd₃

The double-hexagonal close-packed localized moment 5f system UPd₃ exhibits four phase transitions below 8 K. We present new measurements of the magnetic susceptibility of UPd₃ single crystals. The phase transitions are attributed to a sequence of antiferroquadrupolar ordered structures of the U 5f electrons on the quasi-cubic sites.

From a comprehensive analysis of the neutron scattering, x-ray scattering, and bulk property measurements of this system, we have deduced a new crystal field model for UPd₃. This model, which has a doublet ground state on the quasi-cubic sites with a first excited singlet some 4 meV above it, provides a qualitative understanding of the succession of quadrupolar phase transitions and order parameters of UPd₃. Moreover, the new model also explains the excitations observed, by inelastic neutron scattering, in UPd₃ at 2 K.