Crystal Structures of Ni₂MnGa from Density Functional Calculations

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The different crystal structures of ferromagnetic Ni₂MnGa have been calculated using density functional theory (DFT) with special emphasis on the modulated structures 10M and 14M. These are important for understanding the stability of Ni₂MnGa martensites and their functionality as shape-memory materials. The modulated structures have been optimized in the calculations and their properties are discussed in relation to the structures without modulation. The occurrence of the modulated structures is related to the soft TA₂ phonon mode observed in Ni₂MnGa. The latter is related to the specific nesting behavior of the Fermi surface in Ni₂MnGa. Particular shapes of the modulated structures are stabilized by the covalent interaction mediated by the p-orbitals of Ga and d-orbitals of Ni. The role of this interaction becomes clear seen when considering the phonon dispersion spectrum of Ni₂MnGa, where some characteristic anomalies occur in the coupling of acoustical vibrational modes and the optical modes of Ni.