Abstract :
[en] In this thesis, we explore the electronic, dynamic and thermoelectric properties
of different tellurium-based compounds. We perform ab-initio calculations
within the Vienna Ab-initio Simulation Package (VASP) that works in
the framework of Density Functional Theory (DFT). For the thermoelectric
properties, we use the Boltztrap code that solves the Boltzmann Transport
Equations (BTE) for electrons within the Constant Relaxation Time Approximation
(CRTA). This computational package allows us to obtain accurate
values of the Seebeck coefficient as a function of temperature and carrier concentration
(this last with the help of the rigid band approximation). While
for the calculation of the lattice contribution to the thermal conductivity, we
use the ShengBTE code that solves the BTE for phonons iteratively. The first tellurium-based compound that we study is the best room temperature
thermoelectric material, Bi2Te3. We obtain results comparable with
experimental data for the Seebeck coefficient at room temperature and pressure.
Afterwards, we proceed to explore the evolution of the electronic properties
and the thermoelectric performance under pressures up to 5 GPa. We
reproduce the overall trend of the Seebeck coefficient as a function of pressure
for two different values of doping, however, our results do not reproduce the
small improvement found in experiments close to 1 GPa. Nevertheless, we
support the experimental evidence of an Electronic Topological Transition
(ETT) around 2 GPa and we explain this particular behavior. We also perform calculations on the tellurium-based phase-change materials
(GeTe)x(Sb2Te3)1 (with x = 1, 2, 3). We show results for different
stacking configurations since for some compositions, the stacking arrangement
of the atoms in the primitive cell is still unsettled. We find that the
change of the atomic arrangement leads to the systems to go from semiconductors
to metals. We find that the semiconductor arrangements systematically
overestimate the experimental values for the Seebeck coefficient,
whereas the metallic stacking sequences are in very good agreement with the experimental data for the Seebeck coefficient and for the lattice contribution
to the thermal conductivity. We show that (GeTe)x(Sb2Te3)1 materials could
reach values of ZT=0.5 around 600 K with a proper optimization of S with
respect to the carrier concentration. We also report that in the case of x=3,
the most accepted stacking configuration is dynamically unstable, therefore
we proposed another sequence. Finally, we discuss the discrepancies between
our work and recent theoretical reports that claim the existence of a Dirac-cone
like band structure for (GeTe)2(Sb2Te3)1. We explain the conditions
necessary to obtain such electronic topology.
