Synthesis of iodine filled CoSb3 with extremely low thermal conductivity

p-Type iodine filled CoSb3 skutterudite, which is inaccessible under ambient pressure, was synthesized with high pressure synthesis technique. The successful filling of iodine into voids of CoSb3 crystal structure was verified by combined measurements including X-ray diffraction, Raman spectroscopy, and specific heat. The highest filling fraction of 0.79 was achieved for the sample with a nominal composition of I1.2Co4Sb12. Beneficial from such a high filling fraction of iodine and the strong phonon-scattering from iodine fillers, the thermal conductivity reached as low as 0.7 W m−1 K−1 for I0.79Co4Sb12, which is the lowest value among all elemental filled skutterudites. Introducing iodine fillers is thus a practical way to further suppress thermal conductivity for single- and multiple-filled skutterudites.