Polymorphism in K3SbS4 and the Role of Tungsten Substitution on Structure and Potassium-Ion Conductivity

This work revisits the K+-ion-conducting K3–xSb1–xWxS4 (0 ≤ x ≤ 0.10) system and finds a more complex structural scenario than previously reported. K3SbS4 does not only crystallize in the reported Cmc21 space group but also in a structure best described in the R3c space group. Both polymorphs are present at room temperature, whereas variable-temperature synchrotron X-ray diffraction shows that only the R3c phase persists at elevated temperatures. Similarly, the substitution of Sb(+V) with W(+VI) stabilizes the high-temperature R3c polymorph. K3SbS4 further shows similar diffusion pathways for both polymorphs. It matches the measured transport properties dominated by W(+VI) substitution, increasing ionic conductivity up to 0.64 mS·cm–1 at 298 K for nominal K2.92Sb0.92W0.08S4 with an activation energy of 0.29 eV. This work emphasizes the need for a complementary understanding of atomic arrangement and microstructure in potassium-ion conductors.