The magnetic structure at 15 K of LaFe10.8Al2.2 and its ternary nitride LaF
e10.8Al2.2N3 have been determined by high resolution time-of-flight neutron
powder diffraction. Magnetism in these materials is determined by clusters
formed by a central Fe atom surrounded by Fe atoms with icosahedral symmet
ry. This icosahedral symmetry is not affected by N uptake since both compou
nds are confirmed to crystallize in the cubic NaZn13-type structure. The Al
atoms are statistically distributed over the 96i cluster site only, while
N atoms are found to fully occupy the highly symmetrical 24d interstitial h
ole site. Charging with N gives rise to an 8% increase in the unit cell vol
ume of the parent compound. The magnetic structures are overall ferromagnet
ic for both compounds, with each of the two Fe sublattices aligned parallel
to each other. The influence of N uptake on the magnitudes of the cluster
Fe moments will be discussed. (C) 2000 American Institute of Physics. [S002
1-8979(00)40108-8].