Structural and dynamical fluctuations in off-stoichiometric NiMnGa shape-memory alloys

Measurements and modeling of the 3D diffuse scattering from off-stoichiometric NiMnGa shape memory alloys reveal evidence of structural and dynamical precursors to the phase transition. A model of the diffuse scattering in the high temperature cubic L2₁ phase indicates that at temperatures tens of degrees higher than transition temperature, T_c, the lattice exhibits tetragonally distorted local regions that are clear precursors to the phase transition. The model also accounts for lattice deformation caused by precursor nanoregions of the martensite phase and thermal scattering from phonons and agrees well with the observed diffuse scattering maps in reciprocal space. A distinctive feature of the diffuse scattering is that it is highly anisotropic: Around (H0H) reflections, the diffuse scattering is strongly compressed along the [H0H] and enhanced along the [-H0H] direction. Additionally, localized intensity maxima associated with phasons are observed at temperatures 30-50 K above T_c. They clearly demonstrate that each phason corresponds to an individual point in reciprocal space, which is consistent with dynamical phase fluctuations of a well-formed charge density wave resulting from Peierls instability.