Theory-guided discovery of pressure-induced transitions in the fast-ion conductor BaSnF4

Fast-ion conductors such as BaSn⁢F4 are of significant interest for next-generation solid-state battery technologies due to their high ionic conductivity and chemical stability. However, the behavior of these materials under extreme conditions remains poorly understood, despite the relevance of pressure-induced modifications for tuning functional properties. In this study, we combine density functional theory (DFT) calculations with high-pressure experiments to investigate the structural evolution of BaSn⁢F4 up to 40 GPa. DFT predicts two pressure-induced phase transitions: from the ambient-pressure tetragonal 𝑃⁢4/nmm phase to a monoclinic 𝑃⁢21/𝑚-I structure at 10 GPa, and subsequently to a denser monoclinic 𝑃⁢21/m-II phase at 32 GPa. The first transition is experimentally confirmed via angle-dispersive X-ray diffraction, Raman spectroscopy, and electrical resistivity measurements, all performed at ambient temperature. The second transition is supported by distinct changes in high-pressure Raman modes and resistivity behavior, consistent with a further structural reorganization. These findings not only clarify the high-pressure phase diagram of BaSn⁢F4, but also shed light on the potential for pressure-tuned ionic transport in fluorostannate-based solid electrolytes.