First principles calculations show high thermoelectric capability in lanthanum phosphide

Topological insulators are exotic materials that conduct electricity at their surfaces while behaving as insulators in the bulk of their volume. In recent years, scientists have investigated the potential of using TIs as thermoelectric materials, since topological insulators and conventional thermoelectrics share similar characteristics.

In the new paper, Chen et al. investigated the lattice dynamics and thermoelectric properties of lanthanum phosphide (LaP). Using first-principle calculations and the Boltzmann transport theory, they confirmed the dynamic stability of LaP based on phonon dispersion.

Phonons are divided into two branches: acoustical phonons that are low-frequency vibrations related to heat conduction and optical phonons that are high-frequency vibrations with larger scattering rates due to the anharmonicity of phonons.

The researchers showed that the different atomic masses of La and P atoms lead to an acoustic-optical band gap that prohibits scattering between acoustic and optical phonon modes. As a result, the lattice thermal conductivity is low. At the same time, the material possesses a high Seebeck coefficient and high electrical conductivity. Their findings indicate LaP as a promising thermoelectric material.

“Although the acoustic branches dominate heat transport, the contribution of the optical branches is as consequential,” said co-author Xiang-Rong Chen. “This is because the optical branches provide scattering channels for the acoustic modes, which leads to three-phonon scattering that dominates the thermal transport.”

Source: “Thermoelectric properties of topological insulator lanthanum phosphide via first-principles study,” by Yu Zhou, Wang-Li Tao, Zhao-Yi Zeng, Xiang-Rong Chen, and Qi-Feng Chen, Journal of Applied Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5043170 .