Excellent Thermoelectric Performances of Pressure Synthesized ZnSe2

TL;DR

This study demonstrates that pressure-synthesized ZnSe2 exhibits exceptionally low thermal conductivity and promising electrical properties, resulting in high thermoelectric efficiency suitable for energy conversion applications.

Contribution

It provides the first detailed calculation of lattice thermal conductivity and thermoelectric performance of ZnSe2 under pressure, highlighting its potential as a high-performance thermoelectric material.

Findings

Very low thermal conductivity of 0.69 W/(mK) at room temperature.

Both p-type and n-type ZnSe2 show promising electrical transport properties.

High thermoelectric figure of merit achieved for ZnSe2.

Abstract

We calculate the lattice thermal conductivities of the pyrite-type ZnSe2 at pressures of 0 and 10 GPa using the linearized phonon Boltzmann transport equation. We obtain a very low value [0.69 W/(mK) at room temperature at 0 GPa], comparable to the best thermoelectric materials. The vibrational spectrum is characterized by the isolated high-frequency optical phonon modes due to the stretching of Se-Se dimers and low-frequency optical phonon modes due to the rotation of Zn atoms around these dimers. The low-frequency optical phonon modes are characterized by a strong anharmonicity and will substantially increase the three-phonon scattering space which suppress the thermal conductivity. Interestingly, two transverse acoustic phonon modes with similar frequencies and wave vectors have very different degrees of anharmonicity depending on their polarization. We relate this to the low thermal conductivity and show that the anharmonicities of the transverse acoustic phonon modes are connected to the corresponding change in the pyrite parameter, which can be interpreted as a descriptor for the local volume change. To determine the thermoelectric performance of ZnSe2, we also investigate its electrical transport properties. The results show that both p-type or n-type ZnSe2 can show promising electrical transport properties. We trace this back to the complex energy isosurfaces of both valence and conduction bands. The low thermal conductivities and promising electrical transport properties lead to a large thermoelectric figure of merit of ZnSe2 for both p-type and n-type doping.