The interplay of competing magnetic and multipolar interactions in actinide compounds leads to a fascinating variety of ground states exhibiting different types of ordering at low temperatures.  Whilst macroscopic property measurements, such as heat capacity studies, can indicate the existence of phase transitions, microscopic investigations, using techniques such as neutron and x-ray scattering are essential for elucidating the details of the ordered structures.

After a general introduction to these materials, and to the importance of neutron scattering methods, we will show how a combination of macroscopic and microscopic experimental methods have been employed to investigate the series of intermetallic compounds UPd₃, NpPd₃ and PuPd₃ that display quite different ground states.

In UPd₃, the 5f electrons are, unusually, well localised and the system exhibits a sequence of four antiferroquadrupolar ordered phases at low temperatures. We will demonstrate how X-ray resonant scattering studies have allowed us to distinguish between the symmetry allowed structures of the various phases. These ordered phases result from a delicate balance of competing interactions and are extremely sensitive to substitutional alloying on both the U and Pd sites, as revealed in recent measurements.  The magnetic and quadrupolar exchange interactions may be most directly deduced from inelastic neutron scattering studies of the excitation spectra of UPd₃. We shall discuss recent progress in distinguishing these interactions by an analysis of a study of the excitations in single crystal UPd₃ using the MERLIN spectrometer at the UK neutron source ISIS.

NpPd₃ shows two ordered phases at low temperatures, but their order parameters have not yet been determined conclusively.  In contrast, PuPd₃ is antiferromagnetic below 24K.  Recent measurements carried out at the Institute for Transuranium Elements in Karlsruhe, Germany, show that it has a surprisingly high Sommerfeld coefficient at low temperatures which is enhanced by the substitution of Lu.