Metallic grain boundary resistance in the high temperature thermoelectric La3Te4

Electrical resistance from grain boundaries limits the thermoelectric performance of many materials. Boundary resistance is typically observed as an increase in resistivity at low temperature that decays exponentially with temperature like the resistivity of an insulator. As a result, engineering studies to mitigate boundary resistance have focused on low temperature thermoelectrics like Mg₃Sb₂ or improving the average zT of fine-grained, mid-temperature thermoelectrics like half-Heuslers. With less impact at high temperature, there has been little motivation to mitigate boundary resistance in high temperature materials. In this work, we demonstrate that it is necessary to consider grain boundary resistance even in high temperature thermoelectrics by improving thermoelectric performance at temperatures up to 1000 °C in La₃Te₄ by increasing grain size. In contrast with previous reports, this improved performance is largest at high rather than low temperatures. This is a result of a form of boundary resistance not previously reported in thermoelectric materials: uncharged or metallic boundary resistance. We observe a boundary resistance that increases linearly with temperature as expected in metals. With both this new form of boundary resistance and improved high temperature performance in La₃Te₄, we have demonstrated that grain boundary engineering is necessary even for high temperature applications.