Atomic Ordering, Electronic Structure, and Transport Properties of LAST-m Systems
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Atomic Ordering, Electronic Structure, and Transport Properties of LAST-m Systems S. D. Mahanti, Khang Hoang, and Salameh Ahmad 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. These compounds are obtained by starting from PbTe, and replacing pairs of Pb2+ ions by (Ag1+, Sb3+) pairs. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to very low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. Possible origins of this atomic ordering and how the presence of nanostructures affects the electronic structure near the band gap region are discussed. The temperature (T) dependence of electrical conductivity σ (~T-2.2 in the range 300K 0.825, and the second transition is first order. We have looked at the low T structures for several x values. Three basic structures are involved, PbTe (x=0), AgSbTe2 (x=1) and layered AgSbPb2Te4 (x=0.5). The third structure consists of alternate layers of Pb2Te2 and AgSbTe2 (see the left figure in Fig. 3 for x=0.25, where the white layers are Pb2Te2 and the dark layers are AgSbTe2). High
Fig. 3 resolution electron micrograph pictures in LAST-18 and similar systems show layered nanostructures of dimension ~ 5-10 nm embedded in a PbTe matrix.3,7 Selected area diffraction studies show that these inclusions have twice the periodicity of the PbTe lattice.7 Although precise structure of these nano regions still remains an open issue we can tentatively identify them as AgSbPb2Te4 regions. In real systems there will be lattice mismatch between the PbTe
matrix and these layered inclusions. The strain energy will like to create spherical domains (minimize the surface energy) instead of layered domains seen in MC simulation. In addition to the layered structures, MC simulation gives exotic nanostructures for special x values. For x=3/8 we see a sodalite cage-like structure where each cage embeds a PbTe nanocube (see the middle figure in Fig. 3). For x=0.5, we also see a tubular structure (the right figure in Fig. 3). Interestingly in the purely ionic model the tubular and layered structure are degenerate. The degeneracy is likely to be removed when one goes beyond the purely ionic model. Ab initio studies of energetics of defect interaction Clearly the ionic model with a fixed fcc lattice is not the complete picture for LAST-m. In order to go beyond the ionic model we have carried out ab initio energetics study for different values of m and for different cations and anions.8 We will first discuss some of our results for x~0.1, near the LAST-18 region. Physics in the m=0 limit (AgSbTe2) has been published recently.9 For our calculations we take a cubic supercell of 64 atoms (32 Te, 30 Pb, and two defects-
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