Synthesis of (Bi 2 Te 3 ) x (TiTe 2 ) x and (Bi 2 Te 3 ) x (TiTe 2 ) 3 Superlattices

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Synthesis of (Bi2Te3)x(TiTe2)x and (Bi2Te3)x(TiTe2)3 Superlattices M. M. Smeller, Fred R. Harris, and David C. Johnson Materials Science Institute, University of Oregon, 1253 University of Oregon Eugene, OR 97403 U.S.A. ABSTRACT The synthesis of (Bi2Te3)x(TiTe2)x and (Bi2Te3)x(TiTe2)3 superlattices using modulated elemental reactants was successfully accomplished. This required the calibration of the deposition parameters to achieve both the desired atomic compositions of the constituent layers and the deposition of the absolute amounts of each of the components to yield the title compounds. Proper annealing conditions were determined from an investigation of the x-ray diffraction patterns of a superlattice sample as a function of annealing temperature. The change in lattice parameters as a function of x showed the expected linear behavior with slopes consistent with values expected from the published lattice parameters of the binary components. Rietveld refinement showed that the characteristic structure of the binary components is maintained in the superlattices studied. INTRODUCTION Theories predicting the thermal conductivity of superlattices are, by necessity, approximations and not based on first principles [1]. The approximations are generally made to reduce the number of variables in the problem to a manageable level and attempt to conserve the expected largest contributions to the thermal conductivity within a superlattice. Most calculations are based on a solution of the Boltzmann transport equations that is dependant on phonon velocities and lifetimes [2]. The lifetimes used in solving the equation are typically taken from the lifetimes in homogeneous alloys of similar compositions to the superlattices. The solutions based on the homogeneous lifetime are a poor match for experiment because phonon lifetimes vary based on the structure and composition of the material though which the phonon travels. Simkin and Mahan showed that by calculating the mean free path in order to determine lifetime, they could accurately predict thermal conductivities [3]. They held all of the bond spring constants equal and only accounted for mass differences in their calculation. Their calculations matched the experimental results observed for Sb2Te3/Bi2Te3 superlattices in which the thermal conductivity as a function of superlattice period had a minimum value [4]. Since Sb2Te3 and Bi2Te3 possess nearly identical unit cells, approximating the spring constants to be equal is reasonable. In superlattices made of two materials with different lattice structures, the thermal conductivity might be a function of the superlattice period, the relative amounts of the components, and the degree of lattice mismatch between the components. While there are several studies of thermal conductivity as a function of superlattice period, we could not find any studies where the ratio of the components was significantly varied or where any lattice mismatch between components was investigated. To begin to explore these variables, herei

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