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Abstract

In modern materials design there is an increasing interest in computational tools that allow an accurate prediction of material properties at finite temperatures. A key quantity that fully characterizes thermodynamic properties of materials is the Helmholtz free energy of individual structural and magnetic phases. We have therefore performed a systematic study of the capabilities and accuracy of density functional theory (DFT) in determining ab initio free energies for metals. Lattice vibrations, which yield the dominant contribution to the free energy of ordered materials, have been determined within the quasiharmonic approximation. For magnetic materials we have considered magnetic excitation, e.g., in the random phase approximation. An integrated approach, combining electronic, vibrational, and magnetic effects, lead us to an accuracy of a few meV for the free energy up to the melting point of the considered metals. The thus determined free energies have been successfully used to predict martensitic phase transition temperatures. In particular, I present our results for the magentic shape memory alloy Ni₂MnGa, for which we were able to reproduce the complete phase sequence (martensite ↔ pre-martensite ↔ austenite) as a function of temperature. By analyzing the relevant free energy contributions, we also revealed the delicate interplay of phonons and magnons driving the involved phase transitions. We believe that these insights can be used for a systematic search of chemical trends and improvements of the magnetic shape memory properties.