The First Principle Studies Of Several Typical Thermoelectric Materials

With the rapid development of technology and continuous improvement of quantum chemistrycomputational materials science plays an increasingly critical role in the research and developmentfor new materials. The computational materials science not only help us to predict materialproperties and to design new materials with superior performance, but also explain some of thephenomena observed in the experiments. Clathrates or caged compounds have been widely studiedin the past decade to their properties of “Electronic crystal-phonon glass”. Clathrates are composeda three-dimensional framework with large cavities in which guest atoms are filled. The radius ofguest atom is generally less than the radius of the cage. The weak strength of bonding of guest atomand atoms of framework leads to low thermal conductivity due to the phonon scattering of the localvibrations of the guest atoms in the cages. The layer structured In4Se3has very thermal conductivitydue to weak interaction of the van der Waals type between the layers and strong covalent ionicbonding in layers. The large Seebeck coefficient resulting from charge density wave and the lowthermal conductivity makes this compounds attract wide attention.In this thesis, we takes the typical thermoelectric compound of Ba8-xAxCu16P3（0A=Sr, Eu）andIn4X3(X=Te, Se) as our objects. We calculated the electronic bands and density of states withsoftware of WIEN2k, which is written based on density function theory, after structure optimization.We also calculated the transport property coefficients, Seebeck coefficient and electricalconductivity by using program of Boltztrap. We also studied half-metallic properties of newcompounds of Ti2CoAl andTi2CoGa.The main results and conclusions were summarized as follows:(1) The density of states and energy band structures of Ba8Cu16P30were calculated by the WIEN2kbased on the density functional theory. With the output data, we obtained Seebeck coefficientand electrical conductivity by Boltztrap software. The calculated results agree with experimentalresults qualitatively. The higher electrical conductivity suggests a possible improvement inelectrical conductivity could be obtained with appropriate experiment conditions.(2) With replacing Ba atom with Sr atom, a quick energy convergence in Self-consistent calculationcan be obtained. A new compound Sr8Cu16P30is expected to be synthesized under appropriateconditions.(3) With partially replacing Ba atom with Sr atom, asymmetry DOS for up-and down spin suggeststhat EuBa7Cu16P30is ferromagnetic compound. The magnetic moments come from the contribution of4f electrons of Eu atoms.(4) Both In4Te3and In4Se3are direct semiconductors with band gap of0.22eV�'�0.18eV,respectively. With replacing Te atom with Se atom, we find that the Seebeck coefficient changesfrom positive to negative, which suggests change of electrical conduction from P-type to N-type.The temperature dependence of electrical resistivity changes greatly. The calculated results arein agreement with experimental ones qualitatively.(5) In order to study the physical properties of new compound of Ti2CoAl, we built two models forcubic Cu2MnAl-type structures and Hg2CuTi-type structure with space group of Fm3m (No.216）and F43m(No.225). Calculated results after optimization suggest that Hg2CuTi-typestructure with ferromagnetic ground state is preferable due to a lower energy.(6) A half-metallic ferromagnetic nature of Ti2CoAl was confirmed due to the metallic up-spin DOSand semiconducting down-spin DOS with energy gap of0.4eV. The total magnetic moment ofTi2CoAl is2μB.The moments are mainly from Ti atoms，Ti1carrying1.076μBand Ti2carrying0.580μB.