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Connections between Crystallographic Data and New Thermoelectric Compounds

B. C. Sales, B. C. Chakoumakos, and D. Mandrus Solid State Division, Oak Ridge National Laboratory Oak Ridge, TN 37831-6056, U. S. A. ABSTRACT New bulk thermoelectric compounds are normally discovered with the aid of simple qualitative structure-property relationships. Most good thermoelectric materials are narrow gap semiconductors composed of heavy elements with similar electronegativities. The crystal structures are usually of high symmetry (cubic, hexagonal, and possibly tetragonal), and often contain a large number of atoms per unit cell. In the present work a new structure-property relationship is discussed which links atomic displacement parameters (ADPs) and the lattice thermal conductivity of clathrate-like compounds. For many clathrate-like compounds, in which one of the atom-types is weakly bound and “rattles” within its atomic cage, room temperature ADP information can be used to estimate the room temperature lattice thermal conductivity, the vibration frequency of the “rattler”, and the temperature dependence of the heat capacity. ADPs are reported as part of the crystal structure description, and hence APDs represent some of the first information that is known about a new compound. For most ternary and quaternary compounds, all that is known is its crystal structure. ADP information thus provides a useful screening tool for the large and growing crystallographic databases. Examples of the use and limitations of this analysis are presented for several promising classes of thermoelectric materials. INTRODUCTION Atomic displacement parameters (ADPs) measure the mean-square displacement amplitude of an atom about its equilibrium position in a crystal. In the description of a new crystalline compound, crystallographers normally tabulate the room temperature ADP values for each distinct atomic site in the structure1-4. The various ADP values thus comprise some of the first information that is known about a new crystalline compound. The value of the mean square atomic displacement can be due to the vibration of the atom or to static disorder. The effects that this parameter can have on various physical properties, however, have not been widely recognized. In particular, ADPs are not normally used by solid state physicists or chemists as a guide in the search for new compounds with specific properties. ADPs are still regarded by many scientists as unreliable, since in many of the earliest structure determinations, ADPs often became repositories for much of the error in the structure refinement. In addition, crystallographers have not always reported ADP information using a consistent definition, 1 adding further confusion as to the usefulness of ADPs. The purpose of this article is to illustrate that when properly determined, ADPs can be used as a guide in the search for crystalline materials with unusually low lattice thermal conductivities. These materials are of particular interest in the design of thermoelectric compounds with impr