Impact of local lattice distortions on the structural stability of Fe-Pd magnetic shape-memory alloys
The binding surface of Fe-rich Fe-Pd alloys is explored by means of first-principles calculations in the framework of density functional theory involving unconstrained optimization of the atomic positions within a 108-atom supercell. We find that static displacements arising from geometric optimization provide an important contribution to the total energy, effectively compensating favorable contributions gained from introducing L12 order in stoichiometric Fe3Pd. In the concentration range for magnetic shape-memory applications, the energy profile with respect to tetragonal distortion is altered qualitatively, shifting the ground state of the intermixed disordered system from face-centered cubic (fcc) to body-centered tetragonal (bct). From the radial pair distribution function and electronic density of states obtained from a 500-atom supercell calculation we identify the origin of the displacements. These arise from the size-dependent relaxations of the larger Pd atoms, on the fcc side in combination with a Jahn-Teller-like rearrangement of Fe d states at the Fermi level.
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