Acta Crystallographica Section B

Structural Science

Volume 55, Part 3 (June 1999)

research papers

Acta Cryst. (1999). B55, 321-326 [ doi:10.1107/S0108768198016759 ]

Synchrotron X-ray analysis of the electron density in HoFe₂

V. A. Streltsov and N. Ishizawa

Abstract: Structure factors for a small holmium diiron HoFe₂ Laves crystal were measured with focused [lambda] = 0.75 Å synchrotron X-radiation using a fast avalanche photodiode (APD) counter. The deformation electron density ( [Delta] [rho] ) 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 kagomé net hexagons and form, with the Fe atoms, a triangular lattice which is characterized by more stable magnetic order than the kagomé 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 [Delta] [rho] near the atoms along the Ho-Ho, Ho-Fe and Fe-Fe vectors are indications of exchange repulsion. The effect of anharmonicity on the [Delta] [rho] is insignificant.