High-Throughput Screenings And First Principles Studies On New Room Temperature Thermoelectics

In this work,a code based on first-principles calculations and Boltzmann transport equation has been developed.Electrical transport properties of two types of new room temperature thermoelectric materials have been theoretically studied with the package,as well as some high-throughput platforms and related filtering tools.The main contents are as follows.A Fortran code,TransOpt is presented to solve the Boltzmann transport equation.The electronic relaxation time approximation is computed by the recently developed constant electron-phonon coupling approximation.The band structure related scattering phase space is calculated and used in determining the electronic relaxation time.The scattering phase space is treated explicitly with the detailed first-principles electronic structure,while the electron-phonon coupling matrix is kept constant.This constant electron-phonon coupling matrix can be parameterized using the deformation potential method in semiconductors,and the absolute value of electrical conductivity can thus be obtained.This method can accelerate the prediction process of electrical transport properties of materials on the basis of maintaining the accuracy.Thus,it is suitable for Transopt software package integrating into the first principles high-throughput materials database platform developed by us.Transport properties,including the electrical conductivity,Seebeck coefficient,electronic thermal conductivity,Lorenz number,power factor,and electronic fitness t function,can be calculated by TransOpt.The electron group velocities v_nk as a function of the position in the Brillouin zone,k,can be determined in two different ways:1)The momentum matrix method,which naturally avoids the“band crossing”problem,and yields better convergence with the number of first principles k points and 2)The energy derivative method,where v_nk is solved by the first derivative of the energy with respect to the k coordinates,which avoids the need to compute momentum matrix elements.TransOpt is integrated into our first-principles high-throughput platform.The code interfaces with the Vienna ab initio Simulation Package(VASP),which can also use full electron-phonon data from quantum espresso(QE).Based on the TransOpt and high-throughput calculations,we studied the electrical transport properties of two types of new room temperature thermoelectrics,materials with the Rashba spin splitting effect and semimetals.The Rashba effect plays a vital role in electronic structures and related functional properties.The strength of the Rashba effect can be measured by the Rashba parameter?_R;it is desirable to manipulate?_R to control the functional properties.The current work illustrates how?_R can be systematically tuned by doping,taking BiTeI as an example.A five-point-spin-texture method is proposed to efficiently screen doped BiTeI systems with the Rashba effect.Our results show that?_R in doped BiTeI can be manipulated within the range of 0?4.05 e V·(?)by doping different elements.The dopants change?_R by affecting both the spin-orbit coupling strength and band gap.Some dopants with low atomic masses give rise to unexpected and sizable?_R,mainly due to the local strains.The calculated electrical transport properties reveal an optimal?_R range of 2.75?3.55 e V·(?)for maximizing the thermoelectric power factors.?_R thus serves as an effective indicator for high-throughput screening of proper dopants and subsequently reveals a few promising Rashba thermoelectrics.This work demonstrates the feasibility of defect-mediated Rashba engineering for optimizing the thermoelectric properties,as well as for manipulating other spin-related functional properties.The electrical transport properties of another new thermoelectric material,semi-metallic half-Hassler alloy,are also investigated.Semimetal has the advantage of high carrier concentration,which avoids various impurity defects caused by heavy doping in semiconductors.It is naturally beneficial to the experimental synthesis and characterization,and greatly expands the available materials of thermoelectrics.We use the first-principles high-throughput screens the half-Heusler semimetals with potential high thermoelectric properties.The high-throughput filters contain stability rule of materials with low energy,materials without lanthanide series atoms,band gap selection rules,semimetal selection rules,materials with high asymmetric density of states between valence band and conduction band.Based on the filters,we finally find Ag As Mg may have high thermoelectric properties and calculate the electrical transport properties with TransOpt.Ag As Mg is calculated to be suitable for p-type thermoelectric applications.From the perspective of high-throughput methods and materials science,this work provides possible exploration path for the research of new thermoelectric materials and the application of semimetal materials in functional materials.