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Electrical Transport Properties of Rare Earth Doped Pentatellurides Nathan D. Lowhorn, Terry M. Tritt, R. T. Littleton IV, Edward E. Abbott1 and J. W. Kolis1 Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, U.S.A. 1 Department of Chemistry, Clemson University, Clemson, SC 29634, U.S.A. ABSTRACT The transition metal pentatellurides HfTe5 and ZrTe5 exhibit a broad resistive anomaly as a function of temperature. This behavior is also reflected in the thermopower as it changes from a large positive value below room temperature to a large negative value at lower temperatures with the zero crossing corresponding well with the peak temperature of the resistive anomaly. The large values of the thermopower at low temperatures (T 150 K) have made these materials attractive for investigation for potential low temperature thermoelectric applications. The magnitude of the resistive peak and the peak temperature are highly sensitive to doping as well as external influences such as magnetic field and pressure. In this study we examine the effect of doping with various rare earth elements (RE = Ce, Sm and Dy) and the subsequent effects on the electrical resistivity and the thermopower. These results will be discussed in relation to the thermoelectric performance of these materials. INTRODUCTION The transition metal pentatellurides HfTe5 and ZrTe5 are quasi-one dimensional materials consisting of MTe3 (M = Hf, Zr) chains connected by tellurium atoms to form two-dimensional sheets. One of these sheets is illustrated in Figure 1. These sheets are weakly bonded to each other by van der Waal’s forces in the b direction. Samples grow along the a axis forming long, ribbon-like samples.[1] These materials have been studied extensively since they were first grown in 1973 up until the early 1980’s. Early transport measurements revealed the existence of a broad resistive anomaly as a function of temperature with the peak temperature TP ≈ 80 K for HfTe5 and TP ≈ 145 K for ZrTe5.[2,3,4] Thermopower measurements by Jones revealed a large a

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Figure 1. Crystal structure of the transition metal pentatellurides HfTe5 and ZrTe5. Dark circles represent Te atoms; light circles represent Hf or Zr atoms. G5.2.1

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Figure 2. Electrical resistivity and thermopower versus temperature for HfTe5. positive thermopower that changes to a large negative thermopower at lower temperatures with the zero crossing coinciding roughly with TP.[5] The thermopower and resistivity of HfTe5 are plotted as a function of temperature in Figure 2. The peak temperature and magnitude of the resistivity and thermopower vary from sample to sample. This resistive anomaly was originally thought to be a charge-density wave (CDW) transition; however, further measurements seemed to discount this theory. The lack of distinct superlattice spots, nonlinear c