Ab initio study of thermoelectric properties of layered materials

Human activity is overloading our atmosphere with carbon dioxide and other global warming emissions with immense risks to our climate. Green energy harvesting are needed to supply electricity from one or different energy sources present in the environment. One of this sources is thermal energy through the phenomenon called thermoelectricity. The thermoelectric energy harvesting technology exploits the conversion of temperature gradient into electric power. The significant request for thermoelectric energy harvesting is justified by developing new thermoelectric materials and the design of new thermoelectric generator (TEG) devices for many different applications. The fundamental problem in creating efficient thermoelectric materials is that they need to be good at conducting electricity, but not at conducting thermal energy. But in most materials, electrical and thermal conductivity go hand in hand. The aim of this thesis is to describe the methods and results in the field of thermoelectricity. I report a theoretical study of electronic properties, transport coefficients and lattice thermal conductivity for promising thermoelctric materials. The theoretical approach to thermoelectricity is unusually complex as it tackles transport in the two distinct subsystems of electrons and phonons. The calculations are rather difficult and use multifarious methods, each with its peculiar problems and pitfalls. In general, there are ab initio electronic bands calculations, rigid-band Boltzmann-equation calculations of electronic transport including various scattering mechanisms, ab initio or model calculations of phonon transport under several scattering (which in turn involve phonon calculations etc.). Comfortingly, the final product is the ZT figure of merit, a blend of the various ingredients that is relatively insensitive to the fine details. Chapter 1 provides context on the importance of thermoelectricity, the basics of this phenomenon and ways proposed in the literature to improve it. Chapter 2 reports the background of density functional theory used for our calculations while Chapter 3 is about transport phenomena in the Boltzmann Transport Equation framework. Chapter 4 reports the methods and results about the $\beta$-phase of Mg$_3$Sb$_2$. Then in Chapter 5, a study of layered perovskite La$_2$Ti$_2$O$_7$ is presented, and finally, in Chapter 6, a new promising material, the orthorhombic phase of LaSO is studied. The last part contains the conclusion remarks.