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

High phonon velocities, i.e., as measured by the speed of sound (v_s) lead to high lattice thermal conductivity (κ_lat), which is detrimental to thermoelectric performance. Conventional wisdom associates v_s exclusively with structural features such as average atomic mass but not the number of conducting electrons. Here, we demonstrate v_s reduction from electronic doping in eight well-known thermoelectric semiconductors and establish carrier density n_H 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 La_3–xTe₄, we find respective decreases of 16% and ∼20% in v_s when raising the n_H from ∼10¹⁹ to 10²¹ cm^–3, which is sufficient to decrease κ_lat by nearly 50%. Such giant softening effects can account for 25% of the optimized thermoelectric figure of merit (zT_max) in high-performing materials (zT_max > 1) by suppressing total thermal conductivity.