Charge-carrier-mediated lattice softening contributes to high zT in thermoelectric semiconductors

Summary High phonon velocities, i.e., as measured by the speed of sound (vs) lead to high lattice thermal conductivity (κlat), which is detrimental to thermoelectric performance. Conventional wisdom associates vs exclusively with structural features such as average atomic mass but not the number of conducting electrons. Here, we demonstrate vs reduction from electronic doping in eight well-known thermoelectric semiconductors and establish carrier density nH as the main cause for the observed lattice softening by ruling out alternative factors such as changes in density, average atomic mass, and defect formation. In p-type SnTe and n-type La3–xTe4, we find respective decreases of 16% and ∼20% in vs when raising the nH from ∼1019 to 1021 cm–3, which is sufficient to decrease κlat by nearly 50%. Such giant softening effects can account for 25% of the optimized thermoelectric figure of merit (zTmax) in high-performing materials (zTmax > 1) by suppressing total thermal conductivity.