Defect Clustering and Nanostructure Formation in PbTe-based Bulk Thermoelectrics
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1044-U02-08
Defect Clustering and Nanostructure Formation in PbTe-based Bulk Thermoelectrics Khang Hoang, and S. D. Mahanti Department of Physics and Astronomy, Michigan State University, East Lansing, MI, 488242320 ABSTRACT In recent years, LAST-m (AgPbmSbTem+2) and related materials have emerged as potential high performance high temperature thermoelectrics. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700 K. This large ZT is most likely due to the low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. The origin of these nanostructures has been studied using Monte Carlo (MC) simulation of an ionic model and ab initio studies of pair interaction energies. Effects of these substitutions on the band structure near the gap and their implications on transport properties are briefly discussed. INTRODUCTION Recently, there have been considerable experimental and theoretical interests in high temperature high performance thermoelectrics. Examples are: skutterudites [1], PbTe/PbSeTe quantum dots [2], LAST-m [3], and doped transition metal oxides [4]. In this paper, we address some atomic and electronic structure related issues in LAST-m. These systems can be considered as a class of quaternary compounds spanning the binary end PbTe (m=∞) and the ternary end AgSbTe2 (m=0). High ZT values (ZT is the figure of merit of a thermoelectric) [5] have been found in the large m limit (m=18) [3]. These systems take advantage of the good thermoelectric properties of PbTe (ZT~0.8 at 700 K) and improve it by reducing its lattice thermal conductivity by about a factor of 2 at high temperatures. The reason for the reduced lattice thermal conductivity has been ascribed to the existence of 5-10 nm diameter nanoparticles embedded in a primarily PbTe matrix [3,6]. The precise stoichiometry and atomic structure of these nanoparticles are not known and continue to be a point of strong debate. Also it is not known whether the embedding matrix is purely PbTe or it contains finite fraction of Ag and Sb. The ordering of Ag and Sb ions in the host matrix and its effect on the electronic structure is also an open and challenging issue. In the absence of any definite structural information about these nanostructures from experiments, we have theoretically addressed the ordering issue. First we assume an ionic model and represent the LAST-m systems by a Coulomb lattice gas (CLG) on a fcc lattice [7]. Next we use ab initio density functional methods to look at the interaction between different guest atoms (such as Ag, Sb, Bi) in the host PbTe matrix. These studies give us a more realistic picture of the interaction, particularly the role of short-range covalent effects. The next question we address is how do these defects (their concentration is large, ~5%) affect the band structure near the chemical potential. It is known that the origin of the observed narrow band gap (~0.2 eV) in PbTe i
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