4:00pm - 4:30pmInvited
Role of Structural Defects and Soft Phonon Modes in Doped Tin Telluride for Thermoelectric Applications
Indian Institute of Technology Mandi, India
Recently, there has been an increasing scientific interests on the essential role of phonons in thermoelectric (TE) materials. The engineering of phonons is an important strategy to achieve enhanced performances of TE materials. The talk will discuss on the TE performance of SnTe emphasizing on the causes and the effects of structural intricacies and consequent enhancement in TE performance. As a crystalline homologue of PbTe with similar electronic band structure, SnTe has been proposed as environmental friendly option for TE applications [1, 2]. The doping in PbTe  and SnTe  is expected to break the center of inversion symmetry, which in turn develops the impurity driven soft phonon modes. With the appearance of soft phonon mode and anharmonicity in the lattice, low energy phonons are expected to confine, locally. Thus, doping in SnTe affects the band effective mass of the carriers while the point defects disturb the high frequency phonons and the appeared soft phonons disturbs overall phonon group velocity, thus resulting poor lattice thermal conductivity .
Author would like to acknowledge DST-SERB, DAE-BRNS and IIT Mandi for financial support.
 A. Banik and K. Biswas, Journal of Materials Chemistry A 2 (25), 9620-9625 (2014).
 Q. Zhang, B. Liao, Y. Lan, K. Lukas, W. Liu, K. Esfarjani, C. Opeil, D. Broido, G. Chen and Z. Ren, Proceedings of the National Academy of Sciences 110 (33), 13261-13266 (2013).
 S. Lee, K. Esfarjani, T. Luo, J. Zhou, Z. Tian and G. Chen, Nat Commun 5 (2014).
 S. Acharya, J. Pandey and A. Soni, Applied Physics Letters 109 (13), 133904 (2016).
4:30pm - 5:00pmInvited
High Thermoelectric Performance in n-type PbTe-GeTe
Nanyang Technological University, Singapore
Thermoelectric materials, which can convert heat into electricity and generate temperature gradient for cooling purpose, have been extensively studied for clean and reliable energy harvesting systems. The narrow gap semiconductor PbTe is very attractive in mid-temperature range due to its excellent thermoelectric performance. At present, p-type PbTe incorporating endotaxial SrTe nanostructures holds the record in figure of merit ZT of ~ 2.5 at 923 K. However, developing high performing n-type PbTe counterpart is still a big challenge. In this study, a series of n-type Sb-doped and Ge-alloyed PbTe compounds were prepared followed by spark plasma sintering (SPS). Powder X-ray diffraction (PXRD) measurement shows no detectable impurity phases. The lattice thermal conductivity κlat of PbTe was greatly decreased by 46% by the introduction of Ge at Pb atoms, which is as low as 0.62 W m-1 K-1 at 574 K for the Pb0.858Sb0.012Ge0.13Te sample. Consequently, a significantly enhanced ZT of 1.31 is obtained at 624 K.
5:00pm - 5:30pmInvited
Direct Observation of Vast Off-Stoichiometric Defects in Single Crystalline SnSe
Southern University of Science and Technology, China
Single crystalline tin selenide (SnSe) recently emerged as a very promising thermoelectric material for waste heat harvesting and thermoelectric cooling, due to its record high figure of merit ZT in mediate temperature range. The most striking feature of SnSe lies in its extremely low lattice thermal conductivity as ascribed to the anisotropic and highly distorted Sn-Se bonds as well as the giant bond anharmonicity by previous studies, yet no theoretical models so far can give a quantitative explanation to such low a lattice thermal conductivity. In this talk, we presented direct observation of anastonishingly vast number of off-stoichiometric Sn vacancies and Se interstitials, using sophisticated aberration corrected scanning transmission electron microscope; and credited the previously reported ultralow thermal conductivity of the SnSe single crystalline samples partly to their off-stoichiometric feature. To further validate the conclusion, we also synthesized stoichiometric SnSe single crystalline samples, and illustrated that the lattice thermal conductivity is deed much higher as compared with the off-stoichiometric single crystals. The scattering efficiency of individual point defect on heat-carrying phonons was then discussed in the state-of-art Debye-Callaway model.
5:30pm - 5:45pmOral
Multiband Thermoelectric Transport and Native Defects in Mixed-valent Polar Chalcogenides with Complex Crystal Structures
Centre National de la Recherche Scientifique (CNRS), France
Structural complexity can lead towards materials with low thermal conductivities (κ) – one of the key requirements for efficient thermoelectrics. In polar chalcogenides, asymmetric coordination environments and complex structural motifs, e.g. polyanionic networks, layers or channels can be obtained through a charge transfer from the cation to the anionic framework and through stereoactive lone pairs.
Mixed-valent polychalcogenides exhibit a particularly interesting and diverse crystal chemistry due to the formation of homonuclear and often hypervalent bonds between negatively charged chalcogen atoms.[1-3] Many of them possess complex crystal structures, which can result in complex electronic structures and inherently low thermal conductivities. Our studies revealed cases, where a complex crystal structure indeed results in multiple conduction bands with a large valley degeneracy, which enhance the thermoelectric transport properties significantly and where native defects lead to unintentional n-type doping resulting in an almost optimized charge carrier concentrations. Our findings are based on complementary band structure and defect calculations, thermoelectric transport analyses and optical absorption measurements.
 Sheldrick, W. S. Z. Anorg. Allg. Chem. 2012, 638, 2401.
 Mayasree, O.; Sankar, C. R.; Kleinke, K. M.; Kleinke, H. Coord. Chem. Rev. 2012, 256, 1377.
 Böttcher, P. Angew. Chem. Int. Ed. Engl. 1988, 27, 759.
5:45pm - 6:00pmOral
Cu8GeSe6–based Thermoelectric Materials with Argyrodite Structure
1Shanghai Institute of Ceramics, Chinese Academy of Sciences, China; 2University of Chinese Academy of Sciences, China; 3Aarhus University, Denmark; 4Shanghai University, China; 5Shanghai Institute of Materials Genome, China
Liquid-like superionic thermoelectric materials have attracted great attentions recently due to the extremely low lattice thermal conductivity and high thermoelectric figure of merit (ZT). Argyrodite-type compounds are typical superionic semiconductors with two independent structural units that can be used to separately tune electrical and thermal transports. In this work, we report that Cu8GeSe6 with argyrodite structure is a new class of thermoelectric materials with extremely low lattice thermal conductivity. The presence of two independent structural units in Cu8GeSe6 provides the possibility to greatly improve electrical transports while maintaining the ultralow lattice thermal conductivity. Via alloying Ag and Te in Cu8GeSe6, the ZTs are significantly improved to above unity at 800 K in Cu7.6Ag0.4GeSe5.1Te0.9, comparable with the best superionic liquid-like thermoelectric materials. The ultralow thermal conductivity is mainly attributed to the weak chemical bonding between Cu atoms and the rigid [GeSe6] sublattice.