Phonon dispersions of Ni–Mn–Al shape memory alloy

Normal modes of vibration of a Ni–Mn–Al single crystal have been measured by inelastic neutron scattering. The force constants have been fitted to the Born–von Karman model using axially symmetric forces. Dispersion curves of both acoustical and optical phonons have been determined along the high symmetry [1 0 0], [1 1 0] and [1 1 1] directions. The temperature dependence of the normal modes revealed an anomalous softening of the TA2[1 1 0] phonons observed in the range of 0.1–0.25 reciprocal lattice units in good agreement with recent ab initio calculations. Contrary to the acoustical TA2[1 1 0] modes. The optical TO2[1 1 0] modes with the same polarisation showed a normal behaviour with temperature, namely a decrease in frequency upon heating due to increasing anharmonicity. Elastic scattering performed along the [1 1¯ 0] direction did not reveal any significant elastic or diffuse scattering.