Ag Vacancies as "Killer-Defects" in CaAgSb Thermoelectrics

The AMX Zintl compound CaAgSb was recently identified as a promising thermoelectric material with high hole mobility and low lattice thermal conductivity. The single parabolic band model predicts that a zT of ∼1 can be achieved if the carrier concentration can be tuned to ∼10¹⁹ cm^-3. However, the high inherent p-type carrier concentration of ∼10²⁰ cm^-3 in CaAgSb has limited further optimization of zT in p-type samples and has prevented n-type doping. In this work, we use a combination of computational and experimental tools to study the Fermi-level tunability of CaAgSb. Defect calculations based on density functional theory (DFT) reveal that acceptor-type defects, in particular Ag-vacancies, are the dominant defect across the full chemical potential space. This pins the Fermi energy within the valence band, leading to predicted p-type carrier concentrations that fluctuate within a narrow range. Crystal Orbital Hamilton Population (COHP) analysis shows that the Ag-Sb antibonding orbitals lie below the Fermi energy, which may explain the low Ag-vacancy formation energy in CaAgSb. Experimentally, we used a phase boundary mapping approach to explore the defect chemistry under different synthesis conditions. Samples were synthesized in the Ca-rich, Ag-rich, and Sb-rich regions of the phase diagram, and all were found to have high p-type carrier concentrations, ranging from 6.0 × 10¹⁹ to 1.8 × 10²⁰ cm^-3, and therefore similar thermal and electronic properties, consistent with the defect calculations. Taken together, our results confirm that Ag vacancies act as killer defects in CaAgSb, posing the primary challenge for further improvement of thermoelectric performance.