The phase relation of the La-Co-Si ternary system in the La-poor region (≤25 at. % La) was determined by means of x-ray powder diffraction. In this region there exist four ternary compounds: ${\mathrm{LaCoSi}}_3$, ${\mathrm{LaCoSi}}_2$, ${\mathrm{LaCo}}_2$${\mathrm{Si}}_2$, and ${\mathrm{LaCo}}_{13\mathrm{-}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$. The cubic ${\mathrm{LaCo}}_{13\mathrm{-}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ intermetallic compound with ${\mathrm{NaZn}}_{13}$-type structure could be changed to a tetragonal one with ${\mathrm{NaZn}}_{13}$-derivative structure by vacuum annealing after arc melting. The tetragonal ${\mathrm{LaCo}}_{13\mathrm{-}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ compounds can exist stably in the composition range between x=2.0 and 5.0. The space group of the tetragonal ${\mathrm{LaCo}}_{13\mathrm{-}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ intermetallic compound is I4/mcm. The crystal structure of the tetragonal phase was refined by the Rietveld method. Si atoms show a strong preference for occupying a specific crystallographic position in the structure; i.e., atomic ordering occurs. Magnetic measurements indicate that the tetragonal ${\mathrm{LaCo}}_{13\mathrm{-}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ intermetallic compounds are ferromagnetic with Curie temperature higher than 900 K and anisotropy fields of about 10 kOe. The saturation magnetic moment per Co atom in the tetragonal ${\mathrm{LaCo}}_{13\mathrm{-}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ compounds decreases with Si content. The composition dependence of the saturation magnetic moment per Co atom can be satisfactorily explained by the rigid-band model.

• Received 22 July 1994

DOI:https://doi.org/10.1103/PhysRevB.51.60