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Nuclear Density Analysis in Solids. The Case of Ba8Ga16Si30. B. B. IVERSEN,* A. BENTIEN,** A. E. C. PALMQVIST,*** D. BRYAN,**** G. D. STUCKY,**** A. J. SCHULTZ, ***** R. W. HENNING***** * Department of Chemistry, University of Aarhus, Århus, Denmark ** Max Planck Institute for the Chemical Physics of Solids, Dresden, Germany *** Department of Applied Surface Chemistry, Chalmers University of Technology, Göteborg, Sweden **** Department of Chemistry, University of California, Santa Barbara, CA, USA ***** Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL-60439, USA ABSTRACT Multi-temperature (15, 100, 150, 200, 300, 450, 600, 900 K) single crystal neutron diffraction data on the type I clathrate Ba8Ga16Si30 are analyzed with the maximum entropy method to obtain direct space nuclear densities. The nuclear densities suggest that the guest atoms are structurally disordered, and the disorder appears to be temperature dependent with increased host-guest interaction at high temperatures. INTRODUCTION The first family of open framework structures to be scrutinized in a thermoelectric context was the skutterudites [1], but recently also the inorganic clathrates, Figure 1, have been studied with a plethora of techniques [2, 3]. A large number of elemental compositions can form the clathrate type I structure and common to the different compositions is the number of valence electrons (184 e). For this reason chemists often regard the clathrates as Zintl phases, where the guest atoms donate their valence electrons to the framework. However, the Zintl concept has been challenged for the clathrates, since Sr8Ga16Ge30 appear to contain neutral Sr guest atoms [4]. Other studies have focused on the thermal conductivity in the open framework structures with the specific aim of understanding the role of the loosely bound guest atoms for the thermoelectric properties [5]. In this respect well-tested models of thermal vibrations and lattice dynamics in solids have had a remarkable comeback [6]. In a series of papers Sales and coworkers have shown that even the most basic crystallographic parameter, the isotropic mean square displacement parameter Uiso, can provide very useful information on the thermal conductivity of crystalline materials [7]. By using the Debye approximation for the framework atoms surprisingly accurate estimates of the Debye temperature and the thermal conductivity can be obtained even from just the average room temperature value of Uiso. For the guest atoms Uiso provides an estimate of the Einstein temperature for the rattler. Since Uiso is obtained in every crystallographic study, which today amounts to hundreds of thousands, the structure-property relation for the thermal conductivity is a very useful relation, and it is now used to guide search for new thermoelectric materials [7]. However, while standard anisotropic atomic displacement parameters (ADPs) can be obtained in most crystallographic studies, only one study has so far attempted to analyse the thermal motion of a clathrate material i