Ab initio lattice dynamics of Ni2MnX (X = Sn, Sb) magnetic shape memory alloys

In this study, we present the results of first principles calculations of elastic constants and phonon properties of nickel–manganese based magnetic shape memory compounds Ni2MnSn and Ni2MnSb in stoichiometric composition. The plane wave basis sets and pseudopotential method within spin-polarized generalized gradient approximation (σ-GGA) scheme of the density functional theory is applied. In investigation of the phonon dispersion spectra, linear response technique of the Density Functional Perturbation Theory is used. Phonon softening is observed in dispersion spectra at the transverse acoustic mode (TA2) in [ζ ζ 0] direction as an indication of the structural instability of these systems to shear deformation. The vibrational instability of Ni2MnSb system is larger than that of Ni2MnSn yielding negative phonon frequencies. This vibrational anomaly is also verified by the low shear modulus and large elastic anisotropy ratio. The minority spin Fermi surfaces of both systems exhibit strong nesting features.

► Phonon dispersions of Ni2MnSn and Ni2MnSb FMSM systems show softening behavior along [ζ ζ 0] direction.
► The lattice instability is also verified by the low shear modulus and large elastic anisotropy ratio.
► The anomalous vibrations can be attributed to Kohn anomaly and coupling of optical T2g vibrations to acoustical TA2 mode.
► The degree of softening increases with increasing electron concentration.