Precursor effects and premartensitic transformation in Ni2MnGa

Inelastic neutron scattering and neutron diffraction were used to study a single crystal of the Ni2MnGa shape memory Heusler alloy in a wide temperature range covering the parent phase (T>T-1=260 K), a premartensitic phase (T-1>T>T-M), and a martensitic (T<T-M=220 K) phase region. Several anomalies in the phonon dispersion curves in the parent phase were observed. The premartensitic phase involves a transverse modulation of the parent cubic structure with a simple periodicity of (1/3, 1/3, 0). The approximately tetragonal lattice of the low-temperature martensite is distorted by transverse modulations with incommensurate wave vectors (zeta(M), zeta(M), 0) zeta(M) approximate to 0.43. The observed phenomena are attributed to electron-phonon interactions and anharmonic effects.