Abstract :
[en] The goal of this work is to characterize the structure and lattice dynamics of complex
chalcogenide alloys. Particular interest is paid to the system AgPbmSbTem+2
(LAST-m), AgSbTe2 and the binaries PbTe, SnTe and GeTe. Synchrotron radiation
studies including high energy X-ray di raction and nuclear inelastic scattering,
and macroscopic measurements of heat capacity and elastic constants were
performed. A new resonant ultrasound spectroscopy setup with capable of performing
measurements from room temperature to 1073K was built for mechanical
characterization of the thermoelectric alloys at their working temperatures.
The rst chapter presents a brief review of relevant information on thermoelectricity
and on the materials under study. The characterization methods including
heat capacity, resonant ultrasound spectroscopy, X-ray di raction and nuclear
inelastic scattering are introduced. Not as an exhaustive review, but rather in
order to give the reader a basic level of understanding and a sense of the acessible
information. The introduction is followed by three chapters which address the
experimental studies of lattice dynamics in chalcogenide alloys.
Chapter 2 describes the lattice dynamics in the compounds GeTe, SnTe and PbTe
studied by 119Sn and 125Te nuclear inelastic scattering. The obtained partial
density of phonon states were compared with published theoretical calculations,
and the resulting vibrational properties were found to be in good agreement with
these reports. Additionally, the phase purity and structure were characterized by
high energy X-ray di raction. The atomic arrangement, rhombohedral for GeTe
and cubic for SnTe and PbTe, is seem to a ect the density of phonon states,
with the NaCl-type structure having a softer character in comparison with the
rhombohedral structure. In Chapter 3, the lattice dynamics of a polycrystalline AgSbTe2 sample was
investigated by 121Sb and 125Te nuclear inelastic scattering, at low temperatures.
For this compound, the phonon modes have energies below 25meV and
a sound velocity of vs =1490(30) m/s was determined. A simple temperature
independent estimation of the lattice thermal conductivity of AgSbTe2 yielded
L =0.50 0.05Wm1K1. The low Debye temperature, D =150(15)K combined
with the short phonon lifetime and the low sound velocity are found to be
key factors for the low thermal conductivity in AgSbTe2 and are related to the
good thermoelectric performance in AgSbTe2 and AgSbTe2containing systems.
Chapter 4 is dedicated to the study of the average and local structure in bulk
AgPb18SbTe20 alloy, by a combined Rietveld and Pair Distribution Function analysis.
The strong in
uence of the synthesis conditions on the lattice parameters
and on the composition and the concentration of nanoclusters in LAST-18 is con-
rmed. Moreover, the 121Sb and 125Te partial density of phonons states were
obtained by nuclear inelastic scattering in order to separately characterize the
lattice dynamics from the matrix and the nanoinclusions. Additional characterization
of the elastic properties and lattice governed properties were performed
by resonance ultrasound spectroscopy, heat capacity and thermal expansion measurements.
The nal chapter is dedicated to the resonant ultrasound spectroscopy technique,
and the process of building up this bu er-rods high-temperature measurement
system are presented. Advantages and disadvantages, as well as limitations and
di culties are discussed. Using the \mode-tracking" method, the mechanical
behavior of a PbTe and a Niobium sample, from room temperature to 523K and
from room temperature to 973 K, respectively, were investigated.
