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 re-ported 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 & PLUSMN; 2.1 mu m 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 (P3m1) rather than the monoclinic B -type (C2/m) or stable cubic C-type (Ia3). The trigonal Sm2O3 is nano-twinned with twin boundaries of {2110} 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.