Ab initio calculations of structure and lattice dynamics in Ni-Mn-Al shape memory alloys -: art. no. 014111

Ab initio calculations of the magnetic shape-memory alloy Ni50MnxAl50-x with 0less than or equal toxless than or equal to50 were carried out using density functional theory and PAW potentials. The alloy is ferromagnetic in the range from 14 to 31 at.% Mn. The magnetic moment and structural properties are discussed in terms of the density of states. Furthermore martensitic phases with long-periodic shuffling structure were calculated: 2M, 10M, and 14M. Their stability is discussed in respect to the electronic structure and the lattice dynamics, whereby phonon dispersion relations in [110] direction for the cubic L2(1) Heusler structure have been calculated. The transverse-acoustic phonon mode is shown to soften at the wave vector zeta=[1/3 1/3 0] which confirms the tendency of Ni2MnAl to form modulated structures at low temperatures. The theoretical results of the modulated martensitic phases correspond to experimental data, obtained from thin films near the stoichiometric composition.