Ni₂MnGa is a typical example of a Heusler alloy that undergoes a martensitic transformation.
The high-­temperature austenite has a cubic L2₁ structure, whereas below 200 K the structure is orthorhombically distorted. The transformation is completely reversible despite lattice deformations of more than 6% and large strains connected to this change. Ni₂MnGa is attractive for application in actuators and sensors because magnetism can control its phases.
Before one can systematically use the composition to improve the transistion temperature, the phases of the stoichiometric compound need to be understood in detail. Several (modulated) structures have been experimentally observed in the martensite, but their appearance and importance for the shape memory effect are not well known. We, therefore, systematically studied the various phases (austenite, pre-martensite, and both modulated and unmodulated martensite) and their transitions using ab initio DFT (in the GGA approximation and with the PAW method). We calculated phonon spectra in the quasi­harmonic approximation to derive the finite temperature properties. A surprising result is that the soft phonon mode in the cubic structure does not disappear at elevated temperatures. We subsequently used a soft-mode analysis to determine the martensitic and pre­martensite modulations
We checked that the stable modulated structures do not show soft modes and reproduced the observed transition temperatures quite well. We found that the transition from pre­martensite to martensite includes both a decreasing c/a ratio and a change in the modulation.
Finally, the unmodulated martensite competes with the modulated one at very low temperatures.

Keywords: shape memory, free energy, martensitic phase transition, phonon softening, ab initio

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