Structures and magnetic properties of the LaNi5-xFex and LaNi5-xMnx compounds have been investigated using neutron diffraction and first-principles tight-binding-linear-muffin-tin-orbital methods. Both neutron diffraction refinement data and total energy calculations show that the Fe and Mn atoms preferentially occupy the 3g sites in the hexagonal CaCu5-type structure. The calculated magnetic moments of Fe and Ni atoms are of 2.4-2.5μB and 0.2-0.5μB in LaNi5-xFex, respectively. The magnetic structure exhibits more localized moments at Fe atoms in LaNi5-xFex when x ≤ 1.0. Electronic structure calculations indicate that s-conduction electron spin polarization from the Ni or La atoms strongly interacts with Fe(Mn) d-spin moments in LaNi5-xFe(Mn)x (x ≠ 0) compounds, which gives rise to a very large valence transferred hyperfine field on the Ni or La sites. This s-d hybridization may lead to an interaction among magnetic clusters in these kinds of materials and may cause a spin freezing effect at low temperature when the Fe(Mn) content is very low in LaNi5-xFe(Mn)x. Mn atoms show magnetic moments of 3.0μB per atom due to a large exchange splitting in LaNi5-xMnx (x ≠ 0). LaNi4Mn is found to be ferrimagnetic, whereas antiferromagnetic exchange coupling between the Mn atoms is preferred for LaNi3Mn2. Both ferrimagnetic and ferromagnetic exchange interactions between Mn atoms are found in the LaNi2Mn3 compounds. The calculated results are in good agreement with the experimental neutron data
