Turbostratic Disorder in [(Bi 2 Te 3 ) x (TiTe 2 ) y ] Superlattices
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Turbostratic Disorder in [(Bi2Te3)x(TiTe2)y] Superlattices Fred R. Harris, Stacey Standridge, Carolyn Feik, and David C. Johnson* Department of Chemistry, University of Oregon, Eugene, OR 97403 ABSTRACT A family of metastable [(Bi2Te3)x(TiTe2)y] superlattices (where x and y denote the number of layers of each of the two components) was prepared by annealing modulated elemental reactant precursors at temperatures below 280°C. Structural analysis by traditional XRD shows the superlattice structures are highly aligned to the substrate with many orders of diffraction peaks from the c direction of the superlattice. Off-axis diffraction techniques presented in this paper suggest that the order between the layers in the a-b direction of these superlattice structures is turbostratic. In this type of behavior, the quasi two-dimensional crystals of Bi2Te3 and TiTe2 that comprise the superlattice structure lie like a deck of cards thrown in disarray on a surface. An analysis of the diffraction data leading to this conclusion is given.
INTRODUCTION Recent reports of [(Bi2Te3)x(Sb2Te3)y] superlattice structures (where x and y are the number of layers of each component from van der Waals gap (VWG) to van der Waals gap) have shown that by layering two materials in a superlattice, the thermal conductivity values are lower than the bulk values of each of the components alone. This increases their efficiency as thermoelectric materials.[1] While the nature of the thermal conductivity reduction in superlattice structures is not fully understood, the main reduction mechanism suggested by Venkatasubramanian indicates that phonons are scattered off of the periodic interfaces within the superlattice structure.[2] Other models predict a significant lowering of the thermal conductivity due to the increase in Umklapp processes caused by the artificial enlargement of the unit cell.[3] We recently reported the synthesis of [(Bi2Te3)x(TiTe2)y] compounds with superlattice structure.[4] These compounds are made from modulated elemental reactants which are gently annealed to 280°C in an inert atmosphere. During their formation, the alternating layers of this superlattice self-align normal to the substrate. The resulting diffraction patterns of these superlattices show a distinct new c lattice parameter as seen by the pronounced (0 0 l) diffraction maxima (see Figure 1). The XRD patterns were refined using Rietveld analysis to determine the spacing of the layers within the superlattice unit cell, the relative density of the layers, and the abruptness of the transistion between the components in the c direction. This analysis indicated that the ratio of densities between the TiTe2 layers and the Bi2Te3 layers remained fairly consistent with the bulk values for both Bi2Te3 and TiTe2. The layer spacings within each component (both VWG and atomic spacings) were consistent with the layering found in the individual components. The VWG spacing between the superlattice components was found to be 3.2 ± 0.1 Å, which is close to the value observed fo
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