Impact of local lattice distortions on the structural stability of Fe-Pd magnetic shape-memory alloys

Abstract

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 L${1}_{2}$ order in stoichiometric Fe${}_{3}$Pd. 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.