Thermoelectric Properties of Chalcogenides: An Ab-initio Study

Gudeli, Vijay Kumar and V, Kanchana (2016) Thermoelectric Properties of Chalcogenides: An Ab-initio Study. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

The continuous increase in energy demand has lead us to look for alternate energy resources. Ther- moelectric materials are one such, which can convert the waste heat into useful electrical energy, which attracted the attention of many researchers in recent years. In the present thesis, we are interested in investigating the thermoelectric properties of chalcogenides using the first principles calculations. Density functional theory (DFT) has been widely used in condensed matter physics to explore different material properties and is most successful theory in electronic structure calcula- tions. In the present work, we have calculated the electronic structure and thermoelectric properties of FeCh2, Fe2GeCh4, CuXCh2 and SrAgChF (Ch = S, Se, Te; X = Al, Ga) compounds using the state of art density functional theory and Boltzmann transport equation. The binary iron based chalcogenides, FeCh2 have both marcasite (m) and pyrite (p) structures. Among them marcasite FeS2 is stable at ambient conditions, and it undergoes a first-order phase transition to pyrite FeS2 at around 3.7 GPa with a volume collapse of about 3%. In addition, phonon dispersion curves unambiguously indicate marcasite phase to be stable under ambient conditions. Furthermore, we did not observe any phonon softening across the marcasite-to-pyrite transition. The possible reason for the transition was analyzed in the present study, which has not been at- tempted previously. We also find that m-FeSe2 and m-FeTe2 are stable at ambient conditions and no structural transition from marcasite to pyrite is seen under the application of hydrostatic pres- sure. In addition, we have calculated the electronic structure and thermoelectric properties of both marcasite and pyrite FeCh2. We found a high thermopower for both phases, especially with p-type doping, which enabled us to predict that FeS2 might have promising applications as a good thermo- electric material. The polymorphic forms of FeSe2 and FeTe2 are also found to be good candidates for thermoelectric applications. However, hole doped m- FeSe2 appears to be the best among the polymorphic forms of FeSe2 and FeTe2 systems. Further we compare the predicted TE properties of the investigated compounds with the experiments wherever available. We have also calculated the ternary Fe-based thermoelectric properties of olivine-type Fe2GeCh4 (Ch = S, Se and Te) using first principles calculations. The calculated transport properties using the semi-local Boltzmann transport equation reveal very high thermopower for both S and Se-based compounds compared to their Te counterparts. The main reason for this high thermopower is the quasi-flat nature of the bands at the valence and conduction band edges. The calculated thermopower of Fe2GeS4 is in good agreement with the experimental reports at room temperature, with the carrier concentration around 1018 −1019 cm−3 . All the investigated systems show an anisotropic nature in their electrical vii conductivity, resulting in a value lesser of the order of 102 along the ‘a’ axis compared to the ‘b’ and ‘c’ axes. Among the studied compounds, Fe2GeS4 and Fe2GeSe4 emerge as promising candidates with good thermoelectric performance. Further we have investigated the transport properties of Cu-based chalcopyrites CuXCh2 (X = Al, Ga; Ch = S, Se, Te). All the studied compounds appear to be direct band gap semiconductors evaluated based on the Tran-Blaha modified Becke-Johnson potential. The heavy and light band combination found near the valence band maximum (VBM) drive these materials to possess good thermoelectric properties. The thermoelectric properties of ternary chalcopyrite type CuGaTe2 are derived, and a figure of merit ZT = 1.69 is obtained at 950 K for a hole concentration of 3.7 × 1019 cm−3 , in agreement with a recent experimental finding of ZT = 1.4, confirming that CuGaTe2 is a promising thermoelectric material. Apart from this we have also found CuAlTe2 to be more promising, in comparison with CuGaTe2, which is reported to be an efficient thermoelectric material with appreciable figure of merit. Another interesting fact about CuAlTe2 is the comparable thermoelectric properties possessed by both ne and np type carriers, which might attract good device applications and are explained in detail using the electronic structure calculations. Also for CuSbS2 (chalcostibite), a better performance is obtained for np type than for ne type doping. The variation of the thermopower as a function of temperature and concentration suggests that CuSbS2 might be a good thermoelectric material at low temperatures, similar to the iso-structural CuBiS2. We also report calculations of the electronic structure, vibrational and transport properties of the p-type semiconductors, SrAgChF (Ch = S, Se, and Te). We find soft phonons with low frequency optical branches intersecting the acoustic modes below 50 cm−1 , indicative of a material with low thermal conductivity. The bands at and near the valence-band maxima are highly two-dimensional, which leads to high thermopowers even at high carrier concentrations, which is a combination that suggests good thermoelectric performance. These materials may be regarded as bulk realizations of superlattice thermoelectrics. Finally we have given the concluding remarks and have also discussed about the possible future work.

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