The Theoretical Study Of The Thermoelectric Properties Of Sr₅Al₂Sb₆ And CuInX₂（X=S,Se,Te）

With the development of global industrialization and the requirement for the energy, energy is becoming one of the most critical issues, which limits the development and progress of human. Therefore, exploring new environmentally friendly energy materials has attracted increased attention from all over the world. Thermoelectric materials are new environmental friendly energy materials, which can realize the direct conversion between electric energy and heat energy without the environment pollution. Thus, thermoelectric materials play an important role in solving energy crisis. However, the low conversion efficiency and the high production cost limit the wide application of thermoelectric materials. Therefore, it is necessary to find thermoelectric materials with high conversion efficiency and explore method for improving thermoelectric conversion efficiency.Here, we used the first-principles method and the semi-classical Boltzmann method to study the electronic structure and thermoelectric properties of Sr₅Al₂Sb₆ and CuInX₂（X=S, Se, Te）.Zintl compound Sr₅Al₂Sb₆ has a similar crystal structure with Ca₅Al₂Sb₆ which has been proved to be promising thermoelectric material. Considering the relationship between crystal structure, electronic structure and transport properties of materials, we guess that Sr₅Al₂Sb₆ may also has good thermoelectric properties. Thus, we calculated the electronic structure and the thermoelectric properties of Sr₅Al₂Sb₆, and compared them with those of Ca₅Al₂Sb₆. The calculation results show that, the stronger Sb3-Sb3 anti-bonding interaction leads to the smaller band gap of Ca₅Al₂Sb₆ than that of Sr₅Al₂Sb₆. In addition, the doped Sr₅Al₂Sb₆ may have promising thermoelectric properties, and its maximum ZT value is 1.01 when the carrier concentration is 1.26 ×1021cm-3 at 850 K. That is to say, Sr₅Al₂Sb₆ is a promising thermoelectric material.CuInTe₂ has been reported to be a promising thermoelectric material with a ZT of 1.18 at 850 K, which is high among pure thermoelectric materials with diamond-like structure. Because CuInS₂ and CuInSe₂ have the similar electronic structure and the same space group with CuInTe₂, we guess that CuInS₂ and CuInSe₂ may also have the similar thermoelectric properties with CuInTe₂. We used varies of method to calculate the electronic structures of several Cu based compounds, and found that the mBJ + U method can accurately calculate the electronic structures of Cu based compounds. The analysis about the electronic structure and thermoelectronic properties of CuInX₂（X = S, Se, Te） shows that, the combination of heavy and light bands near the valence band edge induces the promising thermoelectric properties of CuInX₂（X=S, Se, Te）. Then we calculated the electronic structure and transport properties of CuGaTe₂ and AgGaTe₂, and compared them with that of CuInTe₂. The result shows that the combination of heavy and light bands near the valence band edge of CuInX₂（X=S, Se, Te） is mainly affected by the hybridization between Cu d and X p electrons. This is helpful for the design of high performance thermoelectric materials.