Impact of thermal vibrations on the stability of the FeSn2 intermetallics

We have performed a combined theoretical and experimental study of FeSn ₂ intermetallics. We were motivated by a scarcity of published data as well as previous theoretical calculations of the antiferromagnetic (AFM) state of FeSn ₂, when this compound was found mechanically unstable due to imaginary-frequency phonons. Addressing both mechanical and thermodynamic stability within density-functional-theory (DFT) calculations, we focused on the AFM state as well as the ferromagnetic (FM) state of FeSn ₂, which were both considered in earlier experiments. In contrast to the previous calculations, we found the AFM FeSn ₂ state mechanically stable (no imaginary-frequency phonons). The same is true for the FM state, which possesses a slightly higher energy than the AFM state. The mechanical stability allowed for assessing the thermodynamic properties within both harmonic approximations as well as computationally much more demanding quasi-harmonic approximation. Interestingly, while the static-lattice formation energy of AFM FeSn ₂ is negative and, therefore, the compound is predicted stable with respect to the decomposition into elemental end-members, phonon-related contributions have a destabilizing impact at low temperatures. Our calculations were complemented by the experimental characterization of Fe-Sn samples, and the experimental FeSn ₂ lattice parameters were found neatly matching the theoretical values.