  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.
