Na Doping in PbTe: Solubility, Band Convergence, Phase Boundary Mapping, and Thermoelectric Properties.

Many monumental breakthroughs in -type PbTe thermoelectrics are driven by optimizing a PbNaTe matrix. However, recent works found that > 0.02 in PbNaTe further improves the thermoelectric figure of merit, , despite being above the expected Na solubility limit. We explain the origins of improved performance from excess Na doping through computation and experiments on PbNaTe with 0.01 ≤ ≤ 0.04. High temperature X-ray diffraction and Hall carrier concentration measurements show enhanced Na solubility at high temperatures when > 0.02 but no improvement in carrier concentration, indicating that Na is entering the lattice but is electrically compensated by high intrinsic defect concentrations. The higher Na concentration leads to band convergence between the light and heavy Σ valence bands in PbTe, suppressing bipolar conduction and increasing the Seebeck coefficient. This results in a high temperature nearing 2 for PbNaTe, ∼25% higher than traditionally reported values for pristine PbTe-Na. Further, we apply a phase diagram approach to explain the origins of increased solubility from excess Na doping and offer strategies for repeatable synthesis of high Na-doped materials. A starting matrix of simple, high performing PbNaTe synthesized following our guidelines may be superior to PbNaTe for continued optimization in -type PbTe materials.