First-principles Calculations Of 1-2-2-type Zintl Phase Thermoelectric Materials

Thermoelectric materials can directly convert thermal energy and electrical energy.Thermoelectric materials can be used for power generation or cooling.Compared with other energy materials,thermoelectric materials have their own advantages,and of course there are disadvantages.At present,the energy conversion efficiency of thermoelectric materials has not yet reached the level of large-scale commercial use,and the low energy conversion efficiency limits the application of thermoelectric materials,so there is an urgent need to find a thermoelectric material with a high ZT value.Zintl-phase thermoelectric materials are a type of typical intermetallic compound,which is composed of positively charged cation and anion frameworks and can achieve charge balance.This type of material fits the hypothesis of"electronic crystal-phonon glass".At present,the most widely studied is the 1-2-2-type Zintl phase thermoelectric material,which is a ternary compound with an atomic ratio of 1:2:2.Its advantages are good chemical tunability,many elements can be combined and prepared,and its complex crystal structure can greatly scatter phonons and reduce thermal conductivity.But for experimenters,with hundreds of 1-2-2-type materials,how to choose a suitable system and what optimization strategy to adopt is a long-standing research problem.This study uses the first-principles calculation method,combined with the semi-classical Boltzmann theory to simulate the 1-2-2-type Zintl phase thermoelectric material.The main results obtained are as follows.Systematic study of the electronic structure of 1-2-2-type compounds with space group P（?）m1 and CaAl₂Si₂ crystal structure,including their crystal field splitting energy and band gap,and the isoenergy surfaces and sums of six typical 1-2-2-type compounds have been calculated.The density of states further analyzes the electronic structure of such compounds.It is found that this type of compound has a special energy band structure.The top of the valence band appears at point G in the first Brillouin zone,and the top of the second valence band also appears at point G in the first Brillouin zone,with varying degrees of occurrence.The phenomenon of energy level splitting.The energy difference between the different energy levels is defined as the crystal field splitting energy.The entire 1-2-2-type Zintl phase thermoelectric material has compounds with positive crystal field splitting energy and negative crystal field splitting energy.Therefore,the solid solution screening rules are proposed,and materials with positive crystal field splitting energy and negative crystal field splitting energy are selected for solid solution to achieve the purpose of orbital degeneration,thereby improving the Seebeck coefficient of the material.In addition,it can be seen from the electronic structure that the n-type performance is better than the p-type performance in a considerable number of 1-2-2-type compounds.And the band gap of the whole system is between 0.1 eV-2.0 eV.Studies have found that the size of the band gap is related to the difference in electronegativity between M-site elements and X-site elements.The greater the difference in electronegativity,the larger the band gap.The eight solid solution systems selected according to the screening rules are simulated,namely the solid solution of A-site,M-site and X-site.The ATAT（Alloy-Theoretic Automated Toolkit）software package is used to simulate the specific alloy structure,and then the structure model with the lowest energy is used for further electronic structure calculations.The study found that in the simulation results,Yb_1-xMg_xZn₂Sb₂,Ca_1-xBa_xMg₂Bi₂,Yb_1-xBa_xMg₂Bi₂,YbCd_2-xMg_xSb₂,CaCd_2-xMg_xSb₂,MgZn_2-xMg_xSb₂ and CaZn_2-xMg_xSb₂ can achieve the effect of energy band degeneration.At present,three systems of YbCd_2-xMg_xSb₂,CaCd_2-xMg_xSb₂ and CaZn_2-xMg_xSb₂ have been experimentally verified,which proves the effectiveness of the screening rule.In other solid solution systems,Ba_1-xCa_xMg₂Bi₂,Yb_1-xMg_xZn₂Sb₂,Ba_1-xYb_xMg₂Bi₂ and MgZn_2-xMg_xSb₂ look forward to further experimental verification.Experimenters can also choose a variety of 1-2-2-type compounds for solid solution experiments according to the screening rules and splitting energy spectrum to improve the efficiency of the experiment.