5f-ligand hybridization

When An is small (well below the Hill limit) and the corresponding 5f-5f overlap is large and/or when the 5f-ligand hybridization is strong, one expects that a relatively broad 5f band is formed, intersected by EF. A relatively low density of the 5f states at EF is reflected in a modest value of the specific heat coefficient y. The magnetic susceptibility x is practically temperature independent, but the values can be somewhat enhanced with respect to the specific heat data if the formula... 

The An-An spacing of these compounds by far exceeds the Hill limit (for structural data see table 3.7). Therefore, the 5f-ligand hybridization is the main mechanism controlling the localization of the 5f states. The behaviour of UX3 compounds can be roughly classified as belonging to three qualitatively different groups ... 

The first two items reflect well the trends in the development of the 5f-ligand hybridization discussed by Koelling et al. (1985) for the UX3 series. 

Other UTX compounds. A brief report on p.SR studies in three other UTX intermetallics is given by Prokes et al. (1996). The materials studied are at the borderline of magnetism, having greatly reduced U moments due to 5f-ligand hybridization (Sechovsky and Havela 1988). 

Discussion Since dv v exceeds the Hill limit in these materials the hybridization with ligand states dominates the mechanisms of the 5f electron delocalization. The d-states are situated well below EF in AnUX3 compounds and therefore they do not hybridize with the 5f states of the actinide atoms. If d-states with relatively high N(E) are present in the valence band (which is undoubtedly the case in UT3 compounds) the strength of the 5f-d hybridization becomes a very critical parameter. It depends mostly on details of the mutual position (overlap) of the d-states and the 5f-states which remain pinned at (or near) EF. 

Primarily it is the weakening of the hybridization as the size of the ligand atom increases, i.e., as one moves down a column in the Periodic Table. This is connected with the increasing distance between the 5f and ligand atoms. 

---