Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex ... - Physics

Thermoelectric properties of chalcogenide based Cu 2+x ZnSn 1-x Se 4
Ch. Raju a , M. Falmbigl b , P. Rogl b , X. Yan c , E. Bauer c , J. Horky d , M. Zehetbauer d ,
and Ramesh Chandra Mallik a*
a
Department of Physics, Indian Institute of Science, Bangalore 560012, India
b
Institute of Physical Chemistry, University of Vienna, Währingerstrasse 42, A-1090 Wien, Austria
c
Institute of Solid State Physics, TU Vienna, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria
d
Research Group Physics of Nanostructures Materials, University of Vienna, Boltzmanngasse 5,
Abstract:
A-1090 Wien, Austria
Quaternary chalcogenide compounds Cu 2+x ZnSn 1-x Se 4 (x=0, 0.025, 0.05, 0.75, 0.1, 0.125, 0.15)
were prepared by solid state synthesis. The structural and phase identification of these
compounds were studied by Rietveld X-ray powder diffraction (XPD) refinement combined with
Electron Probe Micro Analyzes (EPMA). The XPD patterns of all samples showed a stannite
phase isotypic with the tetragonal Cu 2 FeSnS 4 -type including ZnSe as a secondary phase. The
samples with the highest Cu-content (x=0.125, 0.15) contained CuSe and SnSe as secondary
phases in addition to ZnSe. Raman spectroscopy was employed for the phase identification in
order to resolve Cu 2 ZnSnSe 4 and ZnSe phase contents, because XPD patterns of both the phases
overlap. In all samples the presence of vibrational modes corresponding to Cu 2 ZnSnSe 4 were
observed and confirm the stannite phase. The electrical resistivity, Seebeck coefficient and
thermal conductivity measurements were carried out as a function of temperature in the range
300-720K. The temperature dependent electrical resistivity data of the undoped sample
confirmed semiconducting behavior in-between 4.2 and 250 K, whereas the samples with
nominal compositions Cu 2+x ZnSn 1-x Se 4 (x=0.05, 0.15) were metallic in nature. The electrical
resistivity is decreasing with increasing doping concentration except for the sample with the
nominal composition Cu 2.1 ZnSn 0.9 Se 4 , probably due to the presence of a high content of the ZnSe
impurity phase. All the samples show a positive Seebeck coefficient throughout the temperature
range investigated, which indicates that the majority carriers are holes. The total thermal
conductivity and phonon thermal conductivity of the undoped sample was significantly less as
compared to the remaining doped samples and this may be due to the larger electronic
contribution and the presence of a higher content of the ZnSe secondary phase in the doped
samples. The maximum figure of merit zT = 0.3 at 720 K occurs for the sample with the nominal
composition Cu 2.05 ZnSn 0.95 Se 4 , but the improvement in zT is rather small due to the presence of
secondary phases. The attempt to improve thermoelectric properties employing a high-pressure
torsion treatment resulted in an enhancement of zT by 30 % up to 625 K for Cu 2.05 ZnSn 0.95 Se 4 .