Site Occupancy Determination in Th₂Zn₁₇- and TbCu₇-types Sm₂Fe_17-Co Compounds using Synchrotron Resonant Diffraction.

Area of Science:

• Materials Science
• Solid State Chemistry
• Magnetism

Background:

• Samarium-iron (Sm2Fe17) compounds are crucial for high-performance permanent magnets.
• Cobalt substitution is a key strategy to enhance their magnetic properties.
• Thermal treatment influences the crystal structure, yielding either disordered (TbCu7) or ordered (Th2Zn17) types.

Purpose of the Study:

• To determine the composition and cobalt site occupancy in both disordered and ordered Sm2Fe17 structures.
• To investigate the impact of cobalt substitution on the crystallographic arrangement.
• To confirm structural findings using complementary spectroscopic techniques.

Main Methods:

• Synchrotron powder diffraction for Rietveld refinement to determine atomic composition and structure.
• Synchrotron resonant (anomalous) diffraction to precisely locate cobalt atoms within the crystal lattice.
• Mössbauer spectroscopy to corroborate the structural and site occupancy results.

Main Results:

• Both disordered (TbCu7) and ordered (Th2Zn17) structures exhibit the same overall composition with a transition metal-to-rare earth ratio of 8.5.
• Cobalt atoms are confirmed to be absent from the transition metal dumbbell sites in both structural types.
• Rietveld refinement and resonant diffraction data align with Mössbauer spectroscopy findings.

Conclusions:

• Cobalt substitution in Sm2Fe17 compounds does not alter the fundamental composition but influences crystallographic ordering.
• The absence of cobalt from dumbbell sites is a critical finding for understanding structure-property relationships in these magnets.
• The study provides detailed structural insights into cobalt-substituted Sm2Fe17 magnets, essential for optimizing magnetic performance.