First Principle Study On Doping Of Thermoelectric Materials SrAl₂Si₂

Since the discovery of the Seebeck effect and the Peltier effect,thermoelectric materials have been widely concerned.Thermoelectric conversion efficiency is a standard to measure the quality of a thermoelectric device.We usually use the thermoelectric figure of merit ZT to evaluate the material's thermoelectric conversion efficiency.The larger the ZT value of the material,the higer the thermoelectric conversion efficiency.Therefore,improving the ZT value of the material is the direction for researchers.SrAl₂Si₂ is a silicide with pseudogaps at the Fermi level and has many attractive physical properties,but it has low thermoelectric figure of merit at room temperature which hampered its thermoelectric performance.In addition,some literatures demonstrated experimentally that substituting Y onto the Sr sites could improve the thermoelectric properties of SrAl₂Si₂ materials.Based on these,we present a series of first-principles density function calculations on the structural,electronic and optical properties of Sr_1-x-x Y_xAl₂Si₂?x=0,0.25,0.5 and 0.75?materials.For the ternary aluminum silicide AAl₂Si₂,it is very stable because of its double-layer structure.In order to explore the difference between valence compounds and non-electron valence compounds in ternary aluminum silicides,we investigated the difference in crystal structure,electronic and optical properties between the electron valence compound SrAl₂Si₂ and the non-electron valence compound YAl₂Si₂.First,the first-principles methods are used to study the structural,electronic and optical properties of the Y-doped Sr_1-xY_xAl₂Si₂?x=0,0.25,0.5 and 0.75?.Indeed,the structure was compressed evidently for Y-doped SrAl₂Si₂,and a structural transition was observed from trigonal to monoclinic configuration for Sr_0.5Y_0.5Al₂Si₂.Band structure calculations revealed that SrAl₂Si₂ undergo semimetal to metal-like transition and the metallic characteristics was enhanced with increasing Y content from x=0.25,0.5 to 0.75,which is consistent with the density of states?DOS?.Finally,the dielectric function,absorption spectrum,energy-loss spectrum and reflectivity were calculated and analyzed for Y-doped SrAl₂Si₂ crystals,which shows that it is a promising dielectric material and UV-transparent material around the range?18-30 eV?.Secondly,we calculated the structural parameters,Mulliken population,energy band structure,density of states and some optical constants of pure YAl₂Si₂.The study result shows that from SrAl₂Si₂ to YAl₂Si₂ the lattice parameter c gradually decreased,but the lattice constant a did not change significantly.The calculation of the energy band structure shows that compared with SrAl₂Si₂ compound,YAl₂Si₂ has more overlap between the valence bands and the conduction bands,and the inter-layer coupling is also more stronger,so YAl₂Si₂ is more metallic,which means that the thermoelectric performance of YAl₂Si₂ is better.Density of states calculatiosn revealed that the density of states of YAl₂Si₂ at the Fermi level is much larger than that of SrAl₂Si₂,which is consistent with the calculation results of the energy band structure and it also has a great influence on their superconductivity.Finally,the calculation of optical constants shows that the static dielectric constant of YAl₂Si₂ is larger,which indicating that YAl₂Si₂material is a good dielectric material;and in the range of 22 eV²7.8 eV,YAl₂Si₂ has better light transparent,which has guiding values for the discovery and preparation of high transparent materials.Doping can modify the energy band structure,and so affects the electrical transport properties of these materials,which means that good doping can improve the thermoelectric performance of the material.The research on doping modification provides new ideas and theoretical guidance for finding high-performance thermoelectric materials.