Structure and magnetism of R2T2X compounds and their hydrides; comparison of lanthanides and actinides

The group of "221" compounds crystallizing in the Mo 2 FeB 2 structure type comprises more than 400 members known so far. The onset of magnetic moments and their ordering can be studied in the actinide representatives by tuning the 5f-ligand hybridization strength, including detailed investigations of the non-Fermi liquid regime. The mechanism of specific two-ion anisotropy, depending exclusively on the positions of the nearest-neighbor U atoms, can be tested due to the variety of coordinations observed within a single structure type. Possible dimerization of the magnetic structures, representing a practical realization of a Shastry-Sutherland lattice, has been the motivation for several studies of rare-earth based materials, and can be a reason for the tendency to magnetic frustration. Hydrogen absorption, being an additional degree of freedom for tuning the physical properties, was found to be limited to 2 H atoms per f.u. in U-based materials. The strengthening of U magnetism may be attributed to predominant volume expansion and consequent 5f-band narrowing. In several rare-earth based compounds much higher H absorption has been observed (the limit seems to be 8 H/f.u.). The structure reacts either by distortion or amorphization in such cases. H absorption leads to weakening of the magnetic interactions, probably related to the reduced concentration of conduction electrons.