Phase Transformation and Phonon Anomalies in Ni₂MnGa

¹ Physics Department, bld. 510B, Brookhaven National Lab., P.O. Box 5000, Upton, NY 11973-5000, U.S.A.
² Lawrence Livermore National Laboratory, Livermore, California 94550, U.S.A.

Abstract

Inelastic neutron scattering experiments and transmission electron microscopy have been used to study a single crystal of the Ni₂MnGa shape memory Heusler alloy in a wide temperature range covering the parent phase (T > T_I = 265 K), a recently discovered premartensitic (T_I > T > T_M) and martensitic (T < T_M = 220 K) phase regions. A temperature dependent anomaly in the TA₂ phonon dispersion in the parent phase was observed and related to the phase transformations. The premartensitic phase involves a transverse modulation of the parent cubic structure with a simple periodicity of 1/3 [110]. The approximately tetragonal lattice of the low-temperature martensite is distorted by transverse modulations with incommensurate wave vectors [ ζM ζM 0] and [ 2ζM 2ζM 0], ζM ≈0.43. The observed phenomena are attributed to electron-phonon interactions and anharmonic effects.