Hill limit (solid-state)

In solid-state physics, the Hill limit is a critical distance defined in a lattice of actinide or rare-earth atoms. These atoms own partially filled $$4f$$ or $$5f$$ levels in their valence shell and are therefore responsible for the main interaction between each atom and its environment. In this context, the hill limit $$r_H$$ is defined as twice the radius of the $$f$$-orbital. Therefore, if two atoms of the lattice are separate by a distance greater than the Hill limit, the overlap of their $$f$$-orbital becomes negligible. A direct consequence is the absence of hopping for the f electrons, ie their localization on the ion sites of the lattice.

Localized f electrons lead to paramagnetic materials since the remaining unpaired spins are stuck in their orbitals. However, when the rare-earth lattice (or a single atom) is embedded in a metallic one (intermetallic compound), interactions with the conduction band allow the f electrons to move through the lattice even for interatomic distances above the Hill limit.