On the structure of rare-earth sesquioxide Sm₂O₃ in Sm₂Co₁₇-type magnets

The unavoidably formed rare-earth sesquioxide Sm₂O₃ microparticles in Sm₂Co₁₇-type magnets have been reported to deteriorate magnets’ magnetic properties but improve their mechanical and electrical performance. Here, we unveil the distribution, structure and elastic properties of Sm₂O₃ in a high-end Sm₂Co₁₇-type magnet. Results show that Sm₂O₃ microparticles with size of 4.8 ± 2.1 μm randomly disperse in the magnet, and they serve as the preferential sites for segregation of micron-sized Zr-rich inclusions. Based on Kikuchi lines and atomic images, the Sm₂O₃ is indexed to be the metastable trigonal A-type (P3¯m1) rather than the monoclinic B-type (C2/m) or stable cubic C-type (Ia3¯). The trigonal Sm₂O₃ is nano-twinned with twin boundaries of {2¯110} planes. Calculated elastic constants suggest that the trigonal Sm₂O₃ is ductile and could be an effective phase to reduce the brittleness of the magnet. Our work may offer insights for alleviating the trade-off between magnetic and mechanical properties of Sm₂Co₁₇-type magnets.