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First-principles study of lattice instabilities in ferromagnetic L12 Fe3Ni: direct force constants method versus linear response

The phonon-dispersion relations for ferromagnetic Fe3Ni in the L12 structure are computed along the high symmetry directions using two powerful first-principles techniques: direct force constants method, which is used in conjunction with a standard ab-initio method, and the linear response approach that is based entirely on the density-functional-perturbation theory. The two methods have been applied with success to reveal the experimental observations that characterize the Invar problem in Fe3Ni most especially in the [110] direction. They also help to shed more light on the additional softening of the TA mode being speculated in [111] direction.