Phase Segregation and Thermoelectric Properties of AgPb m SbTe m+2 m=2, 4, 6, and 8
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Phase Segregation and Thermoelectric Properties of AgPbmSbTem+2 m=2, 4, 6, and 8 Joseph Sootsmana, Robert Pcioneka, Huijun Kongb, Ctirad Uherb, Mercouri G Kanatzidisa a b
Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA
ABSTRACT The preparation and characterization of the AgPbmSbTem+2 family of compounds with m=2, 4, 6, and 8 is reported. Phase segregation was observed in all of these materials. The lattice thermal conductivity of these samples is low (4 were not measured in that study. The m=8 lattice thermal conductivity is also comparable to those measured previously. The m=10 sample reported [12] has a room temperature lattice thermal conductivity of 1.1 W/mK. The behavior of the lattice thermal conductivity does not follow the 1/T dependence predicted by theory [13] and should be investigated further. LAST-4 LAST-6 LAST-8
1.4 1.2
κlat(W/m-K)
1.0 0.8 0.6 0.4 0.2 0.0 300
400
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Temperature (K)
Figure 4. Lattice thermal conductivity data for the series AgPbmSbTem+2 with m=4, 6, 8.
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ELECTRON MICROSCOPY Transmission electron microscopy can give spatial information about the size, distribution and composition of the phases observed by powder x-ray diffraction. Using energy dispersive spectroscopy in the scanning transmission electron microscope (STEM) mode gives the ability to spatially map the composition within the sample. One of these maps is shown in Figure 5. It is clear that regions rich in silver and antimony exist along with regions which are deficient in Pb. The regions observed, on the order of several hundred nanometers, are evenly dispersed through the sample. When these areas are examined under high magnification it appears that they are actually made up of alternating regions of differing composition. These fluctuations are on the order of several nanometers. A typical image of these compositional fluctuations is shown in Figure 6a. In Figure 6b an area within the Ag, Sb-rich region is shown along with the corresponding fast Fourier transform (FFT) of the two regions within the image. This clearly shows the presence of a phase which has approximately double the unit cell of the average NaCltype lattice (a = 6.141Å) B A
Figure 5. (A) Electron map and (B) corresponding elemental maps for AgPb4SbTe6 showing the clear difference in the Ag-Sb rich region and Pb rich regions. A JEOL 2200FS was used for STEM mapping. A
Ag-Sb rich
B
Figure 6. (A)Transmission electron microscope image showing additional structure within the silver-antimony rich regions and (B) a high resolution image of the boundary between the two phases. The corresponding FFT for the two regions clearly shows a 2-fold supercell which is the second phase.
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CONCLUSIONS AgPbmSbTem+2 materials with m=2, 4, 6, and 8 were prepared and characterized. These systems show phase segregation into components that are best described as AgPbm+xSbTem+x+2 and AgPbm-xSbTem-x+2. Prelimi
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