Polymorphism in K3SbS4 and the Role of Tungsten Substitution on Structure and Potassium-Ion Conductivity
Matthias Hartmann, Phuong Nam Le Pham, Jacob C. Hickey, Alexandra Morscher, Marvin A. Kraft, Max Wacha, Eamonn T. Connolly, Lucy K. Saunders, Jürgen Janek, Wolfgang G. Zeier, Xabier Martinez de Irujo LabaldeAbstract
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.