Thermoelectric properties of rare earth containing type-I Clathrate compound, Dy₈Al₁₆Si₃₀

Abstract

Type-I clathrates are of importance for thermoelectric applications as the cage in these structures can be filled with a anharmonic oscillator that decreases the thermal conductivity. Among the various type-I clathrates, Si-based alloys are of relevance for high temperature application and most importantly because they are made of earth abundant elements. In the present work, Dysprosium (Dy) has been chosen as the anharmonic cage element because of its large mass and small size compared to divalent alkali metal ions. Structural characterization of Dy₈Al₁₆Si₃₀ alloy performed using a combination of x-ray diffraction and Rietveld refinement indicates presence of type-I clathrate phase along with DyAl₂Si₂ and DySi₂ phases even after prolonged annealing. The Seebeck coefficient is found to be positive and increases with increasing temperature both before and after annealing. The resistivity is found to be low, 2–10 μΩm and increases with increasing temperature, a highly doped degenerate semiconducting behavior. The thermal conductivity with dominant phonon contribution has been found to decrease on annealing from an unusually high value of ~100–50 Wm⁻¹K⁻¹. The electronic contribution to thermal conductivity is found to be low by an order of magnitude indicating that the cage structure plays a dominant role in phonon transport.