Structural study of amorphous La(Co_1-xTM_x)₁₃ alloys by X-ray diffraction

The concentration dependence of transition metals Cr and Mn on atomic structures in amorphous La(Co_1-xCr_x)₁₃ and La(Co_1-xMn_x)₁₃ alloys has been studied by X-ray diffraction. The concentrations x are varied from 0.05 to 0.30 and from 0.05 to 0.40 in amorphous La(Co_1-xCr_x)₁₃ and La(Co_1-xMn_x)₁₃ alloys, respectively. The fundamental unit structure is a Co(Cr) icosahedron in the amorphous La(Co_1-xCr_x)₁₃ alloys, which consists of 12Co(Cr) atoms at its vertices and a Co atom at its center. The distribution of the icosahedra around a La atom in the near neighbor region resembles that in the crystalline LaCo₁₃ phase and is gradually changed at longer distance. The atomic structure shows no significant change up to x=0.30 in the amorphous La(Co_1-xCr_x)₁₃ alloys. The atomic structure is also composed of the Co(Mn) icosahedra randomly distributed around a La atom in the amorphous La(Co_1-xMn_x)₁₃ alloy. The atomic distance of the first neighbor Co-Co pairs, however, increases and the icosahedral cluster is gradually deformed with the Mn concentration. The difference of the concentration dependence of the atomic structure between the amorphous La(Co_1-xCr_x)₁₃ and La(Co_1-xMn_x)₁₃ alloys is understood by a balance of the size difference between Cr and Mn atoms, and the allowed size of an atom substituting for Co atoms located at the vertices of the icosahedron without any significant deformation.