A good proxy for the numerical quantity of the calculated force constants is to compare thermalconductivity calculations performed with and without applying symmetrisation to the force constants. Figures S1.3-S1.10 compare thermal-conductivity curves for all the pristine CoSb3 and filled XCo8Sb24 models considered in this work computed with and without force-constant symmetrisation, and values at 300 and 600 K are compared in Table .2. We find a maximum absolute difference of 1.05 % in the two sets of values, which indicates that the calculated force constants have a low level of numerical noise. -Page 8 -Cell IFC Supercell Sampling Mesh Table . Comparison of the calculated lattice thermal conductivity 𝜅 latt of CoSb3 at 𝑇 = 300 and 600 K obtained using three different models, viz. (1) 2 ndand 3 rd -order IFCs computed in a 2 × 2 × 2 expansion and a single unit of the 32-atom conventional cell, (2) 2 nd -and 3 rd -order IFCs computed in a 2 × 2 × 2 expansion and a single unit of the 16-atom primitive cell; and (3) 2 nd -and 3 rd -order IFCs computed in a 2 × 2 × 2 expansion of the 16-atom primitive cell. The 𝜅 latt for the three models were evaluated using the sampling meshes indicated.