Low thermal conductivities and excellent thermoelectric performances of the pyrite-type IIB-VIA2 dichalcogenides: ZnS2, CdS2 and CdSe2

By solving the phonon and electron Boltzmann transport equations, we calculate the thermoelectric properties of three pyrite-type IIB-VIA2 dichalcogenides (ZnS2, CdS2 and CdSe2). The results show that they both have low lattice thermal conductivities and promising electrical transport properties. Comparing their detailed phonon properties with that of FeS2, we find that their low lattice thermal conductivities come from their soft phonon modes and strong anharmonicity resulted by their weak bonds between the metal atoms with the nonmetal atoms. Analysis of their electronic band structures indicates that their promising electrical transport properties are contributed by the non-spherical isoenergy Fermi surface in the valence bands and the large energy valley degeneracies and light carrier effective masses in the conduction bands. Additionally, due to their calculated carrier relaxation times of p-type carriers are larger than that of n-type carriers, their thermopower factors of p-type doping are higher than that of n-type doping. As a result, their figure of merit, ZT values, can reach 1.45, 1.37 and 2.29 for p-type doping, and 1.01, 0.57 and 1.16 for n-type doping, respectively. This means that three IIB-VIA2 dichalcogenides (ZnS2, CdS2 and CdSe2) exhibit both excellent thermoelectric properties for p-type and n-type doping, and their thermoelectric properties of p-type doping are better than n-type doping.