Thermoelectric Properties of Magnesium-Doped Tetrahedrite Cu12-xMgxSb4S13

Tetrahedrites, naturally occurring sulfosalt minerals, have been shown to exhibit peak ZT values close to unity near 700K due to the combination of semiconducting-like properties and extremely low lattice thermal conductivity. A wide range of elements can be substituted into tetrahedrites, each of them affecting the thermoelectric properties. Interestingly, all tetrahedrites reported to date contain exclusively d- and p-block elements of the periodic table. Here, we demonstrate that magnesium, an s-block element, can be introduced in Cu12Sb4S13. We successfully prepared a series of polycrystalline samples Cu12-xMgxSb4S13 with nominal compositions of x=0.5, 1.0, 1.5. Powder x-ray diffraction and chemical mapping confirmed that approximately half of the Mg atoms were incorporated into the tetrahedrite unit cell, while the other half formed electrically insulating MgS precipitates. Thermoelectric properties, measured between 5K and 673K, show that the effect of Mg2+ is similar to that of other aliovalent elements substituting for either Cu or Sb. In particular, increasing the Mg content drives the system closer to a semiconducting behavior, leading to a concomitant increase in the thermopower and electrical resistivity and a decrease in the electronic part of the thermal conductivity. Because these two trends counterbalance each other, the overall effect of Mg on the ZT of Cu12Sb4S13 is found to be marginal with a peak ZT of 0.55 at 673K.