Exsolution Process as a Route toward Extremely Low Thermal Conductivity in Cu12Sb4-xTexS13 Tetrahedrites

Achieving extremely low lattice thermal conductivity is an essential requirement for improving the performance of thermoelectric materials. Engineered nano-structures in bulk materials and the search for complex crystal structures that inherently poorly conduct heat are the two main areas of research being currently pursued to achieve this objective. Tetrahedrites, a class of widely studied minerals, show intrinsically very low thermal conductivity values on the order of 0.5 Wm(-1).K-1 at 300 K, leading to interesting thermoelectric properties around 700 K. Here, we report on the low-temperature transport properties of a series of synthetic tetrahedrites Cu12Sb4-xTexS13 and demonstrate that, at low Te concentrations, an exsolution process sets in near 250 K, resulting in the coexistence of two tetrahedrite phases, likely at the submicrometer length scale. Remarkably, this mechanism triggers a significant reduction of similar to 40% in thermal conductivity, which drops to 0.25 W.m(-1)-K-1 below 200 K. This exceptionally low value ranks these materials among the least conductive structures reported so far. Because this mechanism is not restricted to low temperatures and has also been shown to occur in the ternary tetrahedrite Cu12Sb4S13 around 373 K, these results suggest a new avenue for achieving extremely low lattice thermal conductivity in minerals.