Solid Ion Conductors under Pressure: In Situ Monitoring of the Tetragonal to Cubic Phase Transition of $Na_3SbS_4$ and $Na_3PS_4$

The $Na_3PnS_4 (Pn = P, Sb)$ solid electrolytes are promising candidates for sodium solid-state batteries due to their potential high ionic conductivities. Structural modifications of these materials can induce a tetragonal-to-cubic phase transition, either by increasing temperature or by aliovalent substitutions. In this study, we introduce pressure as an alternative approach to observe the tetragonal-to-cubic phase transition in these materials. In situ synchrotron high-pressure powder X-ray diffraction shows a tetragonal-to-cubic phase transition at pressures of 2.9 GPa for $Na_3SbS_4$ and 14.6 GPa for $Na_3PS_4$. Rietveld refinements and symmetry analysis provide insights into the displacive phase transition mechanism related to the motion of $Na^+$ and the rotation of the $SbS_4^{3–}$ tetrahedra. Density functional theory calculations confirm that the cubic phase becomes thermodynamically favorable under high pressure compared to the tetragonal phase. These findings highlight the importance of high-pressure considerations in tailoring the properties of ionic conductors, an area that remains underexplored.