Magnetic structure of LaFe10.8Al2.2 and LaFe10.8Al2.2N3 cluster compounds

The magnetic structure at 15 K of LaFe10.8Al2.2 and its ternary nitride LaFe10.8Al2.2N3 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 symmetry. This icosahedral symmetry is not affected by N uptake since both compounds 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 hole site. Charging with N gives rise to an 8% increase in the unit cell volume of the parent compound. The magnetic structures are overall ferromagnetic 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.