Pressure Effect of Seebeck Coeffcient for Zinc Doped Tin Clathrates
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Pressure Effect of Seebeck Coefficient for Zinc Doped Tin Clathrates F. Chena , K. L. Stokesa and G. S. Nolasb a Advanced Material Research Institute, University of New Orleans, New Orleans, LA 70108, U.S.A. b Department of Physics, University of South Florida, Tampa, FL 33620, U.S.A. ABSTRACT We measured the temperature dependence of electrical resistance (R) and thermopower (S) of Cs8 Zn4 Sn42 under high pressure up to 1.2 GPa. Both R and |S| at room temperature increased with pressure. We observed gap widening, irreversible |S| increasing under high pressure, which were similar to the behaviors of Cs8 Sn44 . However, the relaxation effect of R for Cs8 Zn4 Sn42 was negligible in contrast with that of Cs8 Sn44 . We found that the power factor S 2 σ (σ: electrical conductivity) near room temperature decreased linearly with pressure. The results suggest that the defects in different forms played an important role in transport properties for tin clathrates under high pressure. INTRODUCTION There were great efforts to find the thermoelectric materials with better performance in the past decades [1–3]. In the systems that convert heat energy into electric energy (Seebeck effect) or vice versa (Peltier effect), the efficiency is determined by a dimensionless 2 thermoelectric figure of merit ZT = Sκσ T (κ: thermal conductivity, T : temperature). Compounds of group IV elements that form into clathrate-type structures possess high Seebeck coefficient and electrical conductivity while their thermal conductivity values are low [4], which are good candidates for “electron crystal, phonon glass” (ECPG) [5]. This has stimulated much research interest in these compounds as potential materials for thermoelectric applications [6–8]. Transport properties [9], compressibility, phase transformations [10, 11] and band structure calculations [12] were studied for Si clathrates under high pressure. Thermopower at room temperature for Ge clathrates were reported to increase almost twice under non-hydrostatic pressure up to 7 GPa [13]. Transport properties of Sn clathrates were reported before [4, 14]. Temperature dependence of thermoelectric properties of Cs8 Sn44 were measured under hydrostatic pressure up to 1.2 GPa [15]. The band gap of Cs8 Sn44 was reported to increase under pressure, and the location of the vacancy sites was proposed to change after application of high pressure [15]. To compare the result, we measured the sample Cs8 Zn4 Sn42 and observed a similar irreversible effect on S. The results confirmed a generalized defect picture in Cs8 An Sn46−n , where A = Zn or vacancy. EXPERIMENTAL DETAILS The samples were made using powder metallurgy techniques [14, 16]. Briefly, stoichiometric amounts of the the constituent material were reacted at high temperatures (550 ◦ C) in an inert atmosphere. The resulting crystals were ground into a fine powder and the structural properties were measured by x-ray and neutron diffraction. For transport measurements,
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the powders were hot pressed inside a graphite die at 380 ◦ C and approximately 0.15 GPa
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