First-Principles Investigations On The Thermoelectric Performance For Cinnabar HgTe

Thermoelectric (TE) materials are functional materials that can directly convert the energy between the heat and electricity depending on migratory energy by cavities and electrons in solids. TE materials have immense values and prospects with many applications in thermoelectric power generation and refrigeration. And they have become a fashion in the research field of materials due to the special functions of the utilization of energy and environmental protection. Thus, the search for high-performance TE materials is a hot subject of current research. The performance of TE materials depends on the dimensionless figure of merit (ZT) given by ZT = (S2σT/κ), where S,σ, T andκare the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively. High-performance of a TE material requires a high ZT with an effort to maximize the power-factor S2σand minimizeκ.High pressure is an effective method to change the positions and the distributions of atoms, and it is able to tune the electronic structure and lattice vibration. High pressure can also have a very large effect on the chemical and physical properties of materials. Thus, a wide range of new compounds with different characteristics occur under high pressure. Furthermore, many solids synthesized at high pressure can be quenched to ambient conditions, where they could be thermodynamically metastable, yet remain indefinitely kinetically stable. Then, it is possible to synthesize new materials under high pressure. Such routes to metastable solids have attracted much attention because many of the most interesting and useful materials are metastable.It is in the starting stage to explore the high performance thermoelectric materials through the high pressure method. Among the II-VI compounds, HgTe, a typical material, have abundant structures under high pressure. The effect of pressure on the phase transition of HgTe has been well documented since the 1960s. But there haven't been deeply experimental and theoretical works on the thermoelectric performance of the high pressure phase in HgTe. In this paper, we present a detailed study of the transport properties for the ZB and cinnabar phases of HgTe using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory. Our results show that N-doped cinnabar HgTe has a significant Seebeck coefficient and electrical conductivity along the z axis than those of the N-doped ZB phase. The resulting ZT values along the z axis of the N-doped cinnabar HgTe are predicted to reach very high values of 0.61 at room temperature and 1.74 at 600 K. Therefore, the current theory suggests that the cinnabar structure of HgTe could be a good thermoelectric material. Furthermore, the current study is a good example to illustrate that high pressure could be an effective way to uncover potential thermoelectric materials with the formation of new matters. Moreover, the crystal and electronic structure of HgTe within the cinnabar structure is analyzed in detail, which could theoretically explain why the cinnabar phase of HgTe has good thermoelectric performance microscopically. We find that the total electronic density of states (DOS) near the band gap of the cinnabar phase is significantly higher than that of the ZB phase. In fact, materials with high Seebeck coefficients are often associated with high DOSs near the Fermi level. Therefore, the current DOS results are mainly responsible for the predicted higher Seebeck coefficients in the cinnabar structure than those in the ZB phase for both P- and N-type materials. Generally speaking, for the good thermoelectric materials the energy distribution of carriers should be narrow and have a high carrier velocity in the direction of the applied field, which is possible in a highly anisotropic system. Our calculation of band structure reveals the anisotropic energy distribution in the lowest conduction band (LCB) and the highest valence band (HVB) of cinnabar structure. There is larger energy dispersion along A-H and M-L directions in energy band plot for P- and N- doped samples, respectively. We also find that there are 12 electron pockets in the LCB contributing to the transport properties. These carrier pockets with high anisotropy ensure a large DOS (Seebeck coefficient) and large group velocities (conductivities) simultaneously. We thus conclude that the anisotropy of the electronic structure for the cinnabar phase originated from its chainlike bonding characters mainly responsible for the good thermoelectric performance, especially in the N-type material, owing to the more anisotropic electron pockets in the LCB. This study reveals for the first time that the cinnabar structure of HgTe could be a good thermoelectric material. Our current findings will inevitably attract experimental and theoretical attentions and stimulate future experimental exploration to fully make use of the high thermoelectric performance of the cinnabar HgTe.