Quantum chemical insights into hexaboride electronic structures: correlations within the boron p -orbital subsystem

The notion of strong electronic correlations arose in the context of d -metal oxides such as NiO but can be exemplified on systems as simple as the H 2 molecule. Here we shed light on correlation effects on B 6 2− clusters as found in M B 6 hexaborides and show that the B 2 p valence electrons are fairly correlated. B 6 -octahedron excitation energies computed for CaB 6 and YbB 6 agree with peak positions found by resonant inelastic x-ray scattering, providing a compelling picture for the latter. Our findings characterize these materials as very peculiar p -electron correlated systems and call for more involved many-body investigations within the whole hexaboride family, both alkaline- and rare-earth compounds, not only for N - but also ( N ± 1)-states defining e. g. band gaps.