Solid Ion Conductors under Pressure: <i>In Situ</i> Monitoring of the Tetragonal to Cubic Phase Transition of Na₃SbS₄ and Na₃PS₄

The Na3PnS4 (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 Na3SbS4 and 14.6 GPa for Na3PS4. Rietveld refinements and symmetry analysis provide insights into the displacive phase transition mechanism related to the motion of Na+ and the rotation of the SbS43– 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.