Synchrotron X-ray analysis of the electron density in HoFe2

Structure factors for a small holmium diiron HoFe2 Laves crystal were measured with focused λ = 0.75 A synchrotron X-radiation using a fast avalanche photo-diode (APD) counter. The deformation electron density (Δρ) maps are remarkable for significant excess electron density midway between the Ho atoms, which is not dissimilar to the peaks attributed to classic 'covalent bonding' in C and Si crystals. These residual electrons accumulate at the centres of the kagome net hexagons and form, with the Fe atoms, a triangular lattice which is characterized by more stable magnetic order than the kagome net. Similar peaks occur along the Ho-Fe and Fe-Fe contacts. These results confirm the hypothesis that 5d electrons of the rare-earth atoms are important in the spin-coupling mechanism for HoFe2-type compounds. The 5d electrons are far less localized than the 4f electrons, and considerable 5d-5d and 5d-3d orbital overlap occurs between neighbouring atoms. Aspherical electron density near the Ho nuclei can be associated with the Ho 4f subshell electrons. Strong depletions of the Δρ near the atoms along the Ho-Ho, Ho-Fe and Fe-Fe vectors are indications of exchange repulsion. The effect of anharmonicity on the Δρ is insignificant. (orig.)