Optical Phonon Decay and Improved Thermoelectric Efficiency in p-Type Mg₃Sb₂

The Mg₃Sb₂-based layered compounds exhibit exceptional thermoelectric properties over a wide temperature range and possess the potential to supplant traditional Bi₂Te₃ modules with reliable and economical Mg₃Sb₂-based thermoelectric devices, contingent upon the availability of a complementary p-type Mg₃Sb₂ material with high thermoelectric efficiency comparable to that of n-type Mg₃Sb₂. We provide a simpler method involving the codoping of monovalent atoms (K and Na) at the Mg site of the Mg₃Sb₂ lattice to improve the thermoelectric performance of p-type Mg₃Sb₂. K-Na codoping results in a peak power factor of around 0.85 mW/mK² at 675 K in Mg_2.97K_0.02Na_0.01Sb₂ by enhancing the carrier concentration at the Fermi level. Furthermore, the temperature-dependent development of the Raman modes substantiates the existence of significant lattice anharmonicity in Mg_2.97K_0.02Na_0.01Sb₂, ascribed to volumetric quasi-anharmonicity and three- and four-phonon interactions. As a result, decay of optical phonon happens at higher temperatures, yielding an exceptionally low lattice thermal conductivity of 0.38 W/mK at 675 K in Mg_2.97K_0.02Na_0.01Sb₂, the lowest recorded in p-type Mg₃Sb₂ materials. The thermoelectric figure of merit for Mg_2.97K_0.02Na_0.01Sb₂ attains 0.9 at 675 K, representing the highest documented value for double-doped p-type Mg₃Sb₂ materials thus far. Our extensive findings validate the optical phonon decay and improvement in thermoelectric efficiency of p-type Mg₃Sb₂ via codoping with monovalent atoms for midtemperature thermoelectric applications.