Alkali-Rich Antiperovskite M3FCh (M = Li, Na; Ch = S, Se, Te): The Role of Anions in Phase Stability and Ionic Transport

To improve ionic conductivity, solid-state electrolytes with polarizable anions that weakly interact with mobile ions have received much attention, a recent example being lithium/sodium-rich antiperovskite M₃HCh (M = Li, Na; Ch = S, Se, Te). Herein, in order to clarify the role of anions in antiperovskites, the M₃FCh family, in which the polarizable H^- anion at the octahedral center is replaced by the ionic F^- anion, is investigated theoretically and experimentally. We unexpectedly found that the stronger attractive interaction between F^- and M⁺ ions does not slow down the M⁺ ion diffusion, with the calculated energy barrier being as low as that of M₃HCh. This fact suggests that the low-frequency rotational phonon modes of the octahedron of cubic M₃FCh (and M₃HCh) are intrinsic to facilitate the fast ionic diffusion. A systematic analysis further reveals a correlation between the tolerance factor t and the ionic transport: as t decreases within the cubic phase, the rotational mode becomes softer, resulting in the reduction of the migration energy. The cubic iodine-doped Li₃FSe has a room-temperature ionic conductivity of 5 × 10^-5 S/cm with a bulk activation energy of 0.18 eV.