Electronic transport properties of thermoelectric Zintl compounds Ca5Al2Sb6 and Ca3AlSb3 : an ab initio study

Zintl phases Ca5Al2Sb6 and Ca3AlSb3 exhibit interesting thermoelectric properties in various experimental studies. These p-type semiconductors act indeed like phonon-glass electron-crystal materials. Their complexity (large unit cells with covalent anionic chains) leads to very low lattice thermal conductivity (0.5 W/mK) while their covalent chains lead to low electrical resistivity. In this dissertation, the electronic transport properties of these two Zintl phases are theoretically investigated using the density functional theory and the Boltzmann transport theory. As a first step, the covalent character of the chains in both compounds is confirmed using electron density difference maps. The band structures are computed with Abinit and are used to calculate the transport coefficients in BoltzTraP. Constant relaxation times of 1e−14s and 4e−14s correctly reproduce the electrical resistivity of Ca5Al2Sb6 and Ca3AlSb3 at high temperature, respectively. The validity of this constant approximation is justified by the constant behaviour of the electrical resistivity in both compounds. The electron scattering mechanisms are therefore studied to understand this constant behaviour and the different doping dependences of the resistivity in both phases. Ionized impurity and acoustic phonon scattering are identified as dominant mechanisms from an experimental fitting. Unlike many other materials, the acoustic phonon contribution is small compared to the ionized impurity contribution, even at high temperature, so that they compensate each other above 650 K. It is precisely this compensation that leads to the constant electrical resistivity and therefore explains the validity of a constant relaxation time. Since acoustic phonon scattering differs the most between the two phases, it is then theoretically studied. The sound velocities (derived from computed elastic constants), the band effective masses and the deformation potential constants related to the Bardeen and Shockley model are computed. This analysis highlights the origins of the different acoustic phonon relaxation times and allows one to understand the different variations of the electrical resistivity with the doping level in Ca5Al2Sb6 and Ca3AlSb3. Finally, the anisotropy in the transport properties in Ca5Al2Sb6 is studied and shows promising results in the direction of the covalent chains. The combination of two flat bands and a very dispersive one around the valence band edge is indeed beneficial as it leads to a large Seebeck coefficient while the electrical resistivity remains low in the direction of the light effective mass. At a doping level of 7e19 cm−3, a figure of merit of 0.41 is reached in this direction while the average value is 0.23. These figures of merit can still be increased by tuning the doping level.