Synchrotron X-ray analysis of the electron density in HoFe2.
Structure factors for a small holmium diiron HoFe(2) Laves crystal were measured with focused lambda = 0.75 Å synchrotron X-radiation using a fast avalanche photodiode (APD) counter. The deformation electron density (Deltarho) 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 HoFe(2)-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 Deltarho near the atoms along the Ho-Ho, Ho-Fe and Fe-Fe vectors are indications of exchange repulsion. The effect of anharmonicity on the Deltarho is insignificant.
Streltsov
VA
Crystallography Centre, The University of Western Australia, Nedlands 6907, Australia. strel@crystal.uwa.edu.au
Ishizawa
N