Zintl Phase Eu₂ZnSb₂ Thermoelectric Material:Theoretical Study Of Electrical Properties

Thermoelectric materials can directly convert environmental heat energy with poor energy quality into a wide range of electrical energy,providing an environmentally friendly and stable solution to the problem of energy exhaustion.Compared with other new clean energy sources,the current energy conversion efficiency of thermoelectric materials is relatively low,which limits its wide utilization prospects,so it is necessary to improve the thermoelectric properties of thermoelectric materials.The Zintl phase Eu₂ZnSb₂ compound has a very high thermal conductivity due to the presence of a large amount of intrinsic Zn vacancies in the crystal lattice,so it has excellent thermoelectric properties and is a highly promising thermoelectric material.Based on the extremely low heat transport performance,the first-principles density functional theory calculation of Eu2ZnSb2is carried out,and the electronic structure is analyzed to improve the electrical transport performance,which is of great significance for improving the overall thermoelectric performance.However,the existence of a large number of intrinsic vacancy defects poses a huge challenge to the theoretical calculation of electronic structures.This paper mainly studies the construction of the Eu₂ZnSb₂ unit cell model,the electronic structure of different models and the calculation of the electrical transport properties of the chain structure model with band gap.Two kinds of Zigzag chain structure models and three Armchair chain structure models are proposed for the experimental phenomenon that only half of the Zn position in the crystal lattice can maintain the same as the hypothetical EuZnSb structure prototype.In the ideal structure,the ZnSb layer structure is a two-dimensional honeycomb plane,and two suitable ZnSb₂ chains can be extracted from the ZnSb layer:Zigzag chain and Armchair.For the model construction of Zn vacancy short-range order and Zn vacancy completely disordered,the required unit cell system is huge,and it is difficult to obtain the electronic structure effectively by the current calculation method.Therefore,only Zigzag and Armchair chain structure models meet the expectations of experimental and theoretical calculations in a properly calculated cell model.Through the first-principles density functional theory calculation,the electronic structure of the Zigzag chain structure model and the Armchair chain structure model are studied,and the energy band structure that correctly reflects the properties of the semiconductor is obtained.After analysis and debugging,the strong correlation f orbital electrons existing in the Eu element in the system are corrected by the combination of DFT+U and mBJ potentials.It is calculated that the electronic structures of all Zigzag chain structure models are metallic,and a Dirac point similar to graphene is obtained at the Fermi level.All of the electronic structures of the Armchair chain structure model have band gaps that exhibit semiconductivity.Based on the Armchair chain structure model with band gap,the semi-classical Boltzmann transport theory is used to calculate and predict the electrical transport properties of the three Armchair chain models.Based on the‘rigid band approximation'hypothesis,the carrier concentration range measured by the control experiment is used to calculate the relationship between the transport properties of the Armchair chain and the temperature at different doping levels.The theoretically calculated trend is basically consistent with the experimental results.Under the condition of fixed temperature,the relationship between the electrical transport performance and the change of chemical potential is calculated.The calculation results predict that the n-type power factor of the three Armchair chain structure models will be much better than that of the p-type thermoelectric material in the temperature range of 300 K to 800 K.