Inelastic neutron scattering experiments and transmission electron mic
roscopy have been used to study a single crystal of the Ni2MnGa shape
memory Heusler alloy in a wide temperature range covering the parent p
hase (T>T-1=265 K), a recently discovered premartensitic (T-1>T>T-M) a
nd martensitic (T<T-M=220 K) phase regions. A temperature dependent an
omaly in the TA(2) phonon dispersion in the parent phase was observed
and related to the phase transformations. The premartensitic phase inv
olves a transverse modulation of the parent cubic structure with a sim
ple periodicity of 1/3 [1 1 0]. The approximately tetragonal lattice o
f the low-temperature martensite is distorted by transverse modulation
s with incommensurate wave vectors [zeta(M) zeta(M) 0] and [2 zeta(M)
2 zeta(M) 0], zeta(M) approximate to 0.43. The observed phenomena are
attributed to electron-phonon interactions and anharmonic effects.