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 Mg2.97K0.02Na0.01Sb2 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 Mg2.97K0.02Na0.01Sb2, 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 Mg2.97K0.02Na0.01Sb2, the lowest recorded in p-type Mg3Sb2 materials. The thermoelectric figure of merit for Mg2.97K0.02Na0.01Sb2 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.