SM1 to SM3

SUPPLEMENTARY MATERIAL Some experimental [110] HAADF images with defects in Ni9Sn2S2 (Figure SM1) reveal Sn free inclusion in the main matrix as shown by the corresponding elemental EDX mapping collected in the red window. It confirms the presence of only Ni and S in these defective regions. ABF [100] image and corresponding over mapping of Ni-S and Sn-S species (left part and right part of Figure SM2, respectively) confirm the chemical elements separation in the atomic layers along the c axis where the S rich layers alternate with heterometallic mixed Ni/Sn layers as expected from the structural model. In the experimental image (central part of SM2), the elemental EDX intensity profiles extracted along the stacking direction c support that separation with S free and S rich alternating layers. The good fitting of the calculated ABF image from the proposed structural model to the experimental image is demonstrated. In figure SM3, the thermal conductivity κ as a function of temperature (black dots) increases with T. This trend reflects the increase with T of the electronic part (κelec) calculated from the Wiedemann-Franz Law as also shown in the figure (blue dots).