Localized dimers drive strong anharmonicity and low lattice thermal conductivity in ZnSe2

We calculate the lattice thermal conductivities of the pyrite-type $\mathrm{Zn}{\mathrm{Se}}_{2}$ at pressures of 0 and 10 GPa by 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 spectrums are characterized by the isolated high-frequency optical phonon modes due to the stretching of Se-Se dimers and the low-frequency optical phonon modes with a strong anharmonicity due to the rattling modes of Zn atoms, especially the rotations of Zn atoms around these dimers. This means that the existence of localized Se-Se dimers leads to the strong anharmonicity of the low-frequency optical phonon modes. Interestingly, two transverse acoustic phonon modes with similar frequencies and wave vectors have very different degrees of anharmonicity. We show that the anharmonicities of the transverse acoustic phonon modes are connected to the corresponding changes in the pyrite parameters. Furthermore, to determine the thermoelectric performances of $\mathrm{Zn}{\mathrm{Se}}_{2}$, we also investigate its electrical transport properties. Our results show that both $p$-type and $n$-type $\mathrm{Zn}{\mathrm{Se}}_{2}$ can possess promising electrical transport properties contributed by 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 $\mathrm{Zn}{\mathrm{Se}}_{2}$ for both $p$-type and $n$-type doping at different pressures. Our study reveals the effect of the localized nonmetallic dimers on the anharmonicity and lattice thermal conductivity in the pyrite-type compound, which can be used to guide researchers to seek promising thermoelectric materials containing nonmetallic dimers.