Neutron power diffraction study of Fe₃Ga_1.7As_0.3 and Fe₃Ga_1.15Sb_0.85 B8₂-type compounds

High-resolution neutron powder diffraction measurements on Fe₃Ga_1.7As_0.3 and Fe₃Ga_1.15Sb_0.85 are reported. Substantial differences are observed in the refined crystal and magnetic structures of the two compounds. The compound Fe₃Ga_1.15Sb_0.85 is a disordered effect structure. B8₂ isotopic with the Ni₂In (p6₃/mmc) structure, whilst Fe₃Ga_1.7As_0.3 is a B8₂ variant ordered superstructure with a doubled basal plane lattice constant of the parent Ni₂In subcell. The Fe site occupancies have been refined for the subcell and the superstructure. A low-temperature magnetic structure, with moments orientated in the hexagonal basal plane, is found to give best agreement with the observed data for both compounds. The refined Fe magnetic moments for the superstructure are higher than those for the subcell.