Theory-Guided Discovery of Pressure-Induced Transitions in Fast-Ion Conductor BaSnF4

TL;DR

This study combines DFT calculations and high-pressure experiments to reveal pressure-induced phase transitions in BaSnF4, enhancing understanding of its structural evolution and ionic transport properties under extreme conditions.

Contribution

It provides the first combined theoretical and experimental analysis of BaSnF4's high-pressure phases, identifying two new structural transitions relevant for solid electrolyte tuning.

Findings

DFT predicts two phase transitions at 10 GPa and 32 GPa.

Experimental confirmation of the first transition via X-ray diffraction, Raman, and resistivity.

Evidence of a second transition from Raman and resistivity data.

Abstract

Fast-ion conductors such as BaSnF4 are of significant interest for next-generation solid-state battery technologies due to their high ionic conductivity and chemical stability. However, the behaviour 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 BaSnF4 up to 40 GPa. DFT predicts two pressure-induced phase transitions: from the ambient-pressure tetragonal P4/nmm phase to a monoclinic P21/m-I structure at 10 GPa, and subsequently to a denser monoclinic P21/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 behaviour, consistent with a further structural reorganization. These findings not only clarify the high-pressure phase diagram of BaSnF4, but also shed light on the potential for pressure-tuned ionic transport in fluorostannate-based solid electrolytes.