Phase transition in the shape memory alloy NiTi described by the SCAN meta-GGA functional.

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Climbing the ladder of density functional approximations has long been proposed to systematically improve the accuracy of first-principles calculations employing the density functional theory (DFT); however, up until now, the Perdew-Burke-Ernzerhof (PBE) functional at the second rung of the ladder, has dominated. Here, we present a study of the martensitic phase transition in NiTi based on abinitio molecular dynamics simulations and thermodynamic integration using the third-rung approximation of the strongly constrained and appropriately normalized (SCAN) meta-generalized gradient approximation (GGA). Although the predicted equilibrium lattice constants and formation enthalpy agree well with experimental data, the martensitic transition temperature (MTT) is overestimated by 94% (or 324K too high), compared with only 22% (77K) overestimation by PBE. The latent heat (q) is severely overestimated by SCAN as well. This deteriorated performance originates from the enlarged energy difference (ΔE) between the austenite and martensite phases, compared with the PBE result. Furthermore, a large variation (over 50meV/atom) in ΔE using different meta-GGAs indicates large variations in computed MTTs (∼400-500 K) and q, i.e., the predicted thermodynamic properties depend sensitively on the choice of meta-GGA. This would pose a serious problem when upgrading DFT calculations to the third rung. One possible solution is to add NiTi as a normsystem so that the revised SCAN meta-GGA could reproduce the PBE results of the relevant energy difference.