Determination of symmetry reduced structures by a soft-phonon analysis in Ni 2 MnGa
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1050-BB03-02
Determination of symmetry reduced structures by a soft-phonon analysis in Ni2MnGa Tilmann Hickel, Matthe A. Uijttewaal, Blazej Grabowski, and Jörg Neugebauer Computational Materials Design, Max-Planck-Insitut für Eisenforschung GmbH, Max-PlanckStr. 1, Düsseldorf, 40237, Germany ABSTRACT The shape memory effect of Ni2MnGa is closely related to the fact that the material undergoes a martensitic phase transition, which results in symmetry reductions and deformations when cooling down. However, there are still substantial uncertainties about the phase diagram in the martensitic phase. Particularly challenging is the determination of those phases, which are characterized by shuffling structures. We have applied density functional theory to this problem, which allows an accurate determination of the potential energy surface as a function of the lattice constants. Based on these results we compute ab initio phonon spectra and discuss in detail how they can be used to extract detailed information about the type of shuffling structures and to systematically and efficiently identify stable atomic configurations. INTRODUCTION Ni2MnGa is a typical example of a Heusler alloy that undergoes a martensitic transformation. In the high-temperature austenitic phase it has a cubic L21 structure, whereas below 200 K the symmetry is reduced by an orthorhombic distortion [1]. Despite lattice deformations of more than 6% and large strains connected to this change, it is completely reversible. Hence, Ni2MnGa serves as a shape memory compound. The fact that Ni2MnGa additionally orders ferromagnetically makes the material particularly attractive for applications as actuators and sensors. Despite the large interest in this material, structural details of the different phases are still a subject of debate. For example, the c/a ratio in the martensitic phase is of decisive relevance for the shape memory effect. Neutron scattering experiments have been performed for the stochiometric compound Ni2MnGa [2] and a value of ≈0.94 down to 20 K has been found. However, ab initio calculations [3] predict the ground state to have a c/a value of ≈1.25. For Nirich Ni2+xMn1-xGa compounds, a c/a ratio >1 has indeed been observed experimentally [4]. Nevertheless, the exact position of the phase boundary between c/a1 has not yet been determined (see Figure 1). It is generally assumed that c/a ratios 1. Remarkably, no local minima for different orthorhombic distortions, in particular with c/a
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