Thermoelectric properties of thallium-filled skutterudites

Crystallographic data, electrical and thermal transport measurements, and h eat-capacity data are reported for several compounds with the filled skutte rudite structure: TlxCo4-yFeySb12 and TlxCo4Sb12-ySny. where 0 <(x,y)< 1 an d x approximate to 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 "rat tling" of the Tl about its equilibrium position. A simple analysis of the A DP's and low-temperature heat-capacity data indicate an Einstein temperatur e for the Tl of 53+/-2 K. The resonant scattering of acoustic phonons by th e 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 a re investigated as a function of the void filling with monovalent Tl and th ey are compared to previous data reported for partial filling with trivalen t rare;earth ions. For comparable filling fractions, the Tl compounds had h igher electron mobilities and a similar depression of the thermal conductiv ity. Unlike the rare-earth skutterudites, however, within experimental erro r there was no evidence of mass-fluctuation scattering and the minimum ther mal conductivity occurred near complete void filling. Ar elevated temperatu res (800 K), the maximum thermoelectric figure of merit, ZT, for n-type sam ples is estimated from room-temperature data to be about 0.8 for both Tl-fi lled and rare-earth-filled Co4Sb12.