Phase Transformation and Phonon Anomalies in Ni2MnGa
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.
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 > TI = 265 K), a recently discovered premartensitic (TI > T > TM) and martensitic (T < TM = 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 . 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.
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