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tions in the filled skutterudites are due mostly to the fraction of the rare-earth sites that remain empty in samples prepared using equilibrium-synthesis methods. A simple semiconductor-transport model successfully reproduces most of the qualitative features of the resistivity and Seebeck data from these materials. By varying the extrinsic carrier concentration in the filled skutterudites, this model yields a maximum value for ZT of 1.4 at 1000 K and a maximum ZT value of 0.3 at 300 K. The filled-skutterudite antimonides have demonstrated the validity of the "electron-crystal, phonon-glass" idea in the design of new thermoelectric materials for operation at elevated temperatures. There are many other crystal structures and compounds that contain atomic cages large enough to incorporate additional atoms. It is believed that the filled-skutterudite antimonides only represent a small fraction of a more general class of "rattling semiconductors" and that some of these materials will undoubtedly have high values for ZT at and below room temperature. Acknowledgments It is a pleasure to acknowledge useful discussions with D.G. Mandrus and G.D. Mahan, and to thank D.P. Norton for carefully reading and editing the manuscript. Research was sponsored in part by a Cooperative Research and Development Agreement with Marlow Industries, and in part by the Division of Materials Sciences, U.S. Department of Energy Contract No. DE-AC0596OR22464. Oak Ridge National Laboratory is managed by the Lockheed-Martin Energy Research Corporation. References 1. D.M. Rowe, ed., CRC Handbook ofThermoelectrics (Chemical Rubber, Boca Raton, FL, 1995). 2. G.D. Mahan, in Solid State Physics, edited
by H. Ehrenreich and F. Spaepen (Academic Press, Inc., New York, 1997). 3. C. Wood, Rep. Prog. Phys. 51 (1988) p. 459. 4. G.D. Mahan, B.C. Sales, a n d J.W. Sharp, Phys. Today (1997) p. 42. 5. B.C. Sales, Current Opinion in Solid State and Materials Sciences 2 (1997) p. 284. 6. G.A. Slack, in CRC Handbook ofThermoelec tries, edited by D.M. Rowe (Chemical Rubber, Boca Raton, FL, 1995) p. 407. 7. Ibid., in Solid State Physics, vol. 34, edited by H. Ehrenreich, F. Seitz, and D. Turnbull (Academic Press, Inc., New York, 1979) p. 1. 8. D.G. Cahill, S.K. Watson, a n d R.O. Pohl, Phys. Rev. B 46 (1992) p. 6131. 9. W. Jeitschko and D.J. Braun, Ada Crystallogr. Sec. B 33 (1977) p. 3401. 10. D J . Braun a n d W. J e i t s c h k o , /. LessCommon Metals 76 (1980) p. 147. 11. Ibid.,}. Solid State Chem. 32 (1980) p. 357. 12. Ibid., ]. Less-Common.Metals 76 (1980) p. 33. 13. B.C. Chakoumakos, private communication. 14. N.T. Stetson, S.M. Kauzlarich, a n d H. Hope. ]. Solid State Chem. 91 (1991) p. 140. 15. L.E. DeLong and G.P. Meisner, Solid State Commun. 53 (1985) p. 119. 16. D.T. Morelli a n d G.P. Meisner, /. Appl. Phys. 77 (1995) p. 3777. 17. G.P. Meisner, M.S. Torikachvili, K.N. Yang, M.B. Maple, and R.P. Guertin, ibid. 57 (1985) p. 3073. 18. M.E. Danebrock, C.B.H. Evers, and W. Jeitschko, /. Phys. Chem. Solids 57 (1996) p. 381. 19. G.P. Meisner, Physica 108B (1
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