Analysis of Nanostructuring in High Figure-of-Merit Ag 1–x Pb m SbTe 2+m Thermoelectric Materials By Bruce A. Cook, Matthew J. Kramer, Joel L. Harringa, Mi-Kyung Han, Duck-Young Chung, and Mercouri G. Kanatzidis* 1. Introduction As concerns mount over the impact of carbon emissions and availability of non-renewable fossil fuels, the urgency for reliable and efficient alternative electrical power generation materials and systems becomes increasingly more obvious. Of the many different types of known thermal-to-electrical conversion pro- cesses, thermoelectricity offers unparalleled reliability, [1–3] and the recent breakthroughs in conversion efficiency [4–7] have heightened both awareness and interest in development of new and improved thermo- electric materials. The efficiency, h, of a thermoelectric material is a function of three materials properties: Seebeck coefficient, S, electrical resistivity, r, and thermal conduc- tivity, k, as given by the expressions ZT ¼ S 2 T =ðr  kÞ (1) h ¼ DT T hot ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZT avg p  1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZT avg p þ T cold T hot (2) where ZT is the material’s dimensionless thermoelectric figure-of-merit, T avg is the average temperature of a material operating between the cold and hot junction tempera- tures of T cold and T hot respectively, and in which k is comprised of an electronic contribution, k e , and a lattice, or phonon contribution, k l . Above the Debye temperature, a material’s phonon spectrum typically displays a broad range of wavelengths. While longer wavelength phonons can be scattered by grain boundary interfaces, the propagation of short wavelength phonons is typically reduced by solid solution alloying. [8] Therefore, there exists a means for the scattering of an additional fraction of heat-carrying phonons through the addition of nanoscale second phase inclusions. Early attempts to reduce thermal conductivity through grain size reduction, [9–11] introduction of insoluble second phases, [12,13] or mass fluctuation alloying [8,14] were frequently frustrated by a deleterious impact on electrical conductivity, so that a significant increase in ZT remained elusive. [15] The issue with second phase additions is not that they fail to perform the intended function; indeed, in most cases a reduction in thermal conductivity is observed. However, because of carrier scattering from their incoherent interfaces, electrical conductivity is generally reduced by a proportional amount. In order to de-couple the beneficial phonon scattering effects from the deleterious carrier scattering effects, the second phase inclusion must possess a coherent interface with the matrix, and or occupy a volume smaller than the electron or hole wavelength. Recently, nanoscale inclusions were discovered in alloys of the form AgPb m SbTe 2þm , or (AgSbTe 2 )(PbTe) m , (LAST-m ¼ lead-antimony-silver-tellurium) that were proposed to correlate FULL PAPER www.afm-journal.de [*] Prof. M. G. Kanatzidis, Dr. M.-K. Han Department of Chemistry, Northwestern University Evanston, IL 60208 (USA) E-mail: m-kanatzidis@northwestern.edu Prof. M. G. Kanatzidis, Dr. D. Y. Chung Material Science Division, Argonne National Laboratory Argonne, IL 60439 (USA) Dr. B. A. Cook, J. L. Harringa Materials and Engineering Physics Program, Ames Laboratory Iowa State University Ames, IA 50011-3020 (USA) Dr. M. J. Kramer Materials Science and Engineering, Iowa State University Ames, IA 50011-3020 (USA) DOI: 10.1002/adfm.200801284 Thermoelectric materials based on quaternary compounds Ag 1x Pb m SbTe 2þm exhibit high dimensionless figure-of-merit values, ranging from 1.5 to 1.7 at 700 K. The primary factor contributing to the high figure of merit is a low lattice thermal conductivity, achieved through nanostructuring during melt solidification. As a consequence of nucleation and growth of a second phase, coherent nanoscale inclusions form throughout the material, which are believed to result in scattering of acoustic phonons while causing only minimal scattering of charge carriers. Here, characterization of the nanosized inclusions in Ag 0.53 Pb 18 Sb 1.2 Te 20 that shows a strong tendency for crystallographic orientation along the {001} planes, with a high degree of lattice strain at the interface, consistent with a coherent interfacial boundary is reported. The inclusions are enriched in Ag relative to the matrix, and seem to adopt a cubic, 96 atom per unit cell Ag 2 Te phase based on the Ti 2 Ni type structure. In-situ high-temperature synchrotron radiation diffraction studies indicated that the inclusions remain thermally stable to at least 800 K. 1254 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19, 1254–1259