Thermoelectric properties of thallium-filled skutterudites

Crystallographic data, electrical and thermal transport measurements, and heat-capacity data are reported for several compounds with the filled skutterudite structure: Tl_xCo_4-yFe_ySb₁₂ and Tl_xCo₄Sb_12-ySn_y, where 0<(x,y)<1 and x≈y. These materials have potential use for thermoelectric power generation. The Tl atomic displacement parameters (ADP’s) are large relative to the other elements in the compounds indicating substantial “rattling” of the Tl about its equilibrium position. A simple analysis of the ADP’s and low-temperature heat-capacity data indicate an Einstein temperature for the Tl of 53±2 K. The resonant scattering of acoustic phonons by the Tl “rattlers” are believed to be the major cause of the rapid decrease in the lattice thermal conductivity as small amounts of Tl are inserted into the voids of the skutterudite structure. Thermal and electrical transport are investigated as a function of the void filling with monovalent Tl and they are compared to previous data reported for partial filling with trivalent rare-earth ions. For comparable filling fractions, the Tl compounds had higher electron mobilities and a similar depression of the thermal conductivity. Unlike the rare-earth skutterudites, however, within experimental error there was no evidence of mass-fluctuation scattering and the minimum thermal conductivity occurred near complete void filling. At elevated temperatures (800 K), the maximum thermoelectric figure of merit, ZT, for n-type samples is estimated from room-temperature data to be about 0.8 for both Tl-filled and rare-earth-filled Co₄Sb₁₂.