Interplay of structural instability and lattice dynamics in Ni2 MnAl

We report on the structural instability of Ni2 MnAl from calorimetry measurements and inelastic neutron scattering. The acoustic and optical phonon dispersions along the high-symmetry [ξ00], [ξξ0], and [ξξξ] directions have been interpolated from the normal modes of vibration using the Born-von Kármán model. The tendency of Ni2 MnAl to undergo a martensitic transformation shows up in the anomalous phonon softening of the particular TA2 [ξξ0] phonons in the ξ range 0.1-0.25 rlu of two different crystals. The phonon frequencies of this branch scale inversely with the valence electron concentration in good agreement with ab initio calculations. Contrary to the prediction of first-principles calculations in the Heusler L 21 -Ni2 MnAl, no anomaly is seen in the optical phonons measured in B2 -Ni2 MnAl. The anomalous TA2 [ξξ0] phonon softening is not enhanced below room temperature when Ni2 MnAl orders in the antiferromagnetic state.