The Influence of Mobile Copper Ions on the Glass-Like Thermal Conductivity of Copper-Rich Tetrahedrites

Tetrahedrites are promising <i>p</i>-type thermoelectric materials for energy recovery. We present here the first investigation of the structure and thermoelectric properties of copper-rich tetrahedrites, Cu_12+<i>x</i>Sb₄S₁₃ (0 < <i>x</i> ≤ 2.0). At room temperature, all samples with <i>x</i> > 0 consist of two tetrahedrite phases. <i>In situ</i> neutron diffraction data demonstrate that on heating, the two tetrahedrite phases coalesce into a single tetrahedrite phase at temperatures between 493 and 553 K and that this transition shows marked hysteresis on cooling. Our structural data indicate that copper ions become mobile above 393 K. Marked changes in the temperature dependence of the electrical and thermal transport properties of the copper-rich phases occur at the onset of copper mobility. Excess copper leads to a significant reduction in the total thermal conductivity, which for the nominal composition Cu₁₄Sb₄S₁₃ reaches a value as low as 0.44 W m^–1 K^–1 at room temperature, and to thermoelectric properties consistent with phonon liquid electron crystal (PLEC) behavior