Structures of Sm-Cu intermetallics with Fe as subphase candidates in SmFe12-based permanent magnets studied by first-principles thermodynamics

Since crystalline grain-boundary subphases are expected in SmFe12-based permanent magnets, stable crystal structures dependent on temperatures and compositions are examined by first-principles thermodynamics. The free-energy landscape of ternary Sm-Cu-Fe is constructed with self-consistent phonon theory to overcome the dynamical instability problem, where a barrier between binary B2 Sm-Cu and bcc Fe is found. Cu-rich B2 Sm-Cu is substantially stabilized by phonons as well as by configurational entropy. Furthermore, Fe atoms in the Cu sublattice of B27 SmCu contribute to the relative phase stabilization. These results indicate the existence of B2-B27 two-phase equilibria expanding the stable composition region of nonmagnetic SmCu-based intermetallics.

Automated summary

The stable crystal structures of SmFe12-based permanent magnets, which are expected to have crystalline grain-boundary subphases, are examined by first-principles thermodynamics. A free-energy landscape of ternary Sm-Cu-Fe is constructed, overcoming the dynamical instability problem, to identify stable crystal structures dependent on temperatures and compositions. Cu-rich B2 Sm-Cu is substantially stabilized by both phonons and configurational entropy, and Fe atoms in the Cu sublattice of B27 SmCu contribute to the phase stabilization. These findings expand the stable composition region of nonmagnetic SmCu-based intermetallics by revealing the existence of B2-B27 two-phase equilibria.